The only reason to deny felons a vote is political malfeasance.
Even inmates should generally be able to and encouraged to vote. Civic participation, social buy in, rehabilitation, nothing bad can come of that, these are supposed to be the point.
The fact that these people are subject to our laws is the same reason they should never lose their voice in it, because then you're just subject to the system's whims. The thing that makes their incarceration just in theory to begin with is that they are part of it rather than simply an unwilling kidnapping by power.
To me its disgusting how punitive we are towards our largest prison population on earth. We as a people seem to revel in what we take away from them, even things that might encourage them to be part of society again. We set people who make mistakes, usually because of desperate circumstances to begin with, up to fail in perpetuity, and seem to discourage any effort to rejoin society in good faith.