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First, not all dinosaurs were huge. It's not a trait of dinosaurs in general. Rather, the environmental factors in the past and some factors that are true for reptiles, allowed being huge as an acceptable evolutionary niche, more than today!
Why would some of them grow so big:
In evolution, it's always a bit of a hen and egg problem. And there is between species competition called "Red Queen Hypothesis".
So a likely explanation is that, due to high CO2 atmosphere, plants grew larger which lead to having a long neck or being tall being an advantage. And for carnivorous species bigger herbivores meant that being bigger is an advantage. That, again, meant their prey had pressure to grow bigger (and/or faster), and so on and on.
How could some of them grow that much:
Dinosaurs are reptiles, so they were poikilothermic. Since temperatures have been higher and more stable at their time, a bigger body allowed to keep body temperature stable as well. It doesn't cool off as fast which allowed more activity which allowed eating more which allowed a bigger body.
There was also significantly more oxygen in the atmosphere which is associated with bigger growth in all species since our metabolism depends on it.
This is especially true for Arthropoda btw, some of them were huge in the time of dinosaurs because they breath through their exosceleton. The biggest centipede found (yet) was 2.5 m long! The difference in size between insects in the past and insects today is much bigger than between reptiles today and in the past. All due to bigger plants and more oxygen and the interaction spiral between prey and predator.