I haven’t seen any of these movies but from a purely technical standpoint it would be cheaper to render a CG movie at a lower frame rate.

Movies like this aren’t rendered in real time like a video game. We’re certainly getting a lot closer to that as video game fidelity has started to catch up to CG movies. But, just as a for instance, at the time that Big Hero 6 came out the computer Disney built to render it was the second most powerful computer on the planet and it still took weeks to render.