I'm going to offer an answer because the top responses are either bad or just totally miss the mark. The later responses are just thoughtless kneejerk drivel. It's worth asking and addressing this question because it's impact on electoralism are very material, and it's not for no reason that you made your observation; it's also not necessarily a bad thing that some politicians might be held to a higher standard than others.
The question is:
Why is Kamala being held to a higher standard is the question.
But "higher" than what?
Was Joe Biden being held to a "higher" standard? Do Democrats in general get held to "higher" standards? Rotate the question and it's "Why do we hold politicians to standards?" Are Republicans held to any kind of standards?
First, I think we need to focus on on if politicians are being held to standards by their voters. I won't be addressing any kind of cross party criticism in that I want to focus in on the issue of "potential voters" and if those voters hold a candidate that there is some chance they might vote for to some standards ( or litmus, whatever. The first step will be to address if voters hold politicians to standards (not yours or mine necessarily, but the voters own standards).
Second, I'll address the "higher-ness" of these standards. Are they relative or absolute? Are the candidate or party specific? Do they change?
Finally, I'll address the impacts of this and what it has done over the previous several election cycles.
I think we can look at the recent case of Joe Bidens candidacy to address if Harris is unique in being held to a standard. Joe Biden started campaigning in earnest in around October/ November 2023. This came at the same time as the horrible October 7th massacre that galvanized world support for Israel in the face of a terrorist threat. Joe Biden had historically been the most oro-isralli politician of any party, long before his bid for even vice president.
Relatively quickly after the massacre it was clear that Israel was in no way operating in good faith for their purported goal of recovering hostages. It was obvious that they were targeting journalists and iad workers, that there bombing was indiscriminate and focused on population centers and infrastructure for maximum destruction, and that this was in support of the broader colonial mission that Israel has been in since it's inception.
Because of this, during the primary process, a movement of voters set a standard: that they could not vote for Joe Biden in good conscience based on the manner in which the US was relating to and supporting Israels now obvious extermination policy regarding Gaza. Biden failed that standard with regards to the Undecided movement, and it had cost him the election long before his disasterous debate performance. Joe Biden had been floundering in the polls well before that debate. Because of this, Biden lost his position as candidate, explicitly because the voters had a standard to which the candidate did not meet.
Democratic voters are not the only ones who hold standards for their politicians. Consider the case of a post DJT electoral landscape for Republicans. A Republican candidate basically could not get through a primary not towing the MAGA line (even if this hurt their chances later in the election. So even if they are not your or mine standards, Republicans too are held to standards by their voters (if even we find those standards abhorrent). It's important to understand that in fact these voters do have standards, they just aren't your standards, and they do hold their politicians to them.
To conclude section one: all politicians get held to standards. One of the most important politicians of the modern era just had his career ended by not meeting some of them. Both bases of voters have standards which are different and unique to that bae, and both bases hold their candidates to them accordingly.
Now we come to the question of "higher-ness". Do bases hold these officials to standards equally? Or is there some sliding scale or uniqueness into the way things are applied. I plan on withholding discussion if the consequences of this to the final section, but I am not disregarding it's importance.
In 2008, Barack Obama led one of the most historic campaigns of all time, under the twin banners of "Hope" and "Change". The iconography of Obama's 2008 campaign has and will continue to represent a high water mark for political symbolism and its use in electoralism. Barrack ran as a left-wing populist and won his presidency accordingly. However, once in office, Obama struck a decidedly more rightwing/ centrist stance, effectively governing from the center right. His principal legislation was a Heritage Foundation piece of legislation, a lift and shift of what Romney had implemented in Massachusetts, the ACA, effectively ensconcing private control of health care in the US. While voters did give Obama his second term, there was a distinct feeling of a bait and switch from the populist times of 2008 Obama to the 2012 technocratic, neoliberal approach with which he governed.
This set the stage for 2016, where the demand for "hope and change" was now stronger than ever. Obama has failed to deliver in his messaging and this created opportunities for both right wing and lefty wing populist candidates: Bernie Sanders and Donald Trump. The voter recognized that they had not held Obama to a sufficiently high standard, and because of this, they raised the bar (each group according to their own principles). On the left we saw purity tests around no corporate pac money and a wide suite of progressive litmus. On the right we saw "america first" and isolationism. Both the left and the right bases established a higher standard. However, due to reasons behind the scope of this discussion, only the right were able to effectively hold their candidate to that now higher standard (their standards, internally defined). Leaving the era of technocracy and neoliberalism, it was the candidate held to a higher standard that won that election.
Now examine 2020. Now firmly in the populist era, it was a race on both the left and the right to meet the associated higher standards now established by voters. The Democratic candidates had maybe the most progressive across the board suite of policies ever. And likewise in the right, excepting that they be regressive polocies. Both suites representative of the now higher standards required by their associated blocs. This time however, the more established political players had caught the buzz and responded accordingly. While the DnC was ratfucky as ever, Biden did adopt almost all of those higher standard policy positions to gather those voters into his coalition. His campaign identified the higher standard, held themselves to it, and they won.
Now we have the opportunity to draw some conclusions. There is a "higherness" to standards but they are not necessarily specific to politicians so much as the demographics and voting blocs those politicians draw from. Standards have changed and increased in scrutiny. Standards are also bloc specific, so it doesn't make sense to compare standards across blocs: why a progressive chooses to vote or not vote and why a conservative chooses to vote or not vote are wildly different motivating concerns.
Do standards matter? Objectively yes. Holding candidates to a "higher standard" has represented a winning strategy for voters for the previous 20 years. While those standards are relative depending on which voting block you are courting, the general rule has been that the candidate coming closest to a higher standard wins. This is largely due to a shift away from the kind of technocratic allure of neoliberalism and the general shift towards populism we've seen in the past 20 years: it is distinctly the case that we reside in a new political hedgemony.
Because of the, the prior generation politics of "we know better. just elect the "best" and that should do" has died, and been replaced by stronger willed voters who have specific demands of their politicians. Those politicians who can read and come closest to those demands are the most likely to win: this is an age of populism. Both the MAGA and progressive movement didn't their roots in the technocratic neoliberal approaches emphasizing credentials and expertise over the will of the voter. This was effectively the political hedgemony of the US from about 1967-1978 until 2000-2012. That was the era of technocratic neoliberalism, which has been fully replaced with populism of two distinct variants (which is typical in populist eras).
The idea that voters shouldn't hold their politicians to standards and should just vote for "the one who knows best" is a residual trapping of a previous hedgemony, and extends to almost all aspects of political life and policy.
Holding politicians to higher has also been a demonstrably effective strategy for getting them into office. In the current political hedgemony, the politician held to and able to meet a higher standard typically wins. In this way a "higher standard" benefits both the politician and the electorate. By extending a higher standard for Harris, we're making her a more electable candidate. It doesn't hurt her to be held to a higher standard so long as she strives to meet that standard. It's good for her as a candidate and good for her electorate in that they get closer to accomplishing their political goals.