There might not be performance issues right now (but really, there is on older phones, even my iPhone 6, I can't imagine a 5), but the things they plan to release in the future might be too much. Even the weather effects might be a bit much for some phones.
64 bits are useful for some calculations and big numbers. These can still be made in 32 bits, but the CPU must execute more steps and break it down in parts*, which can really bog down the CPU if there is a lot. They might be adding more stuff to the game, meaning more calculations, maybe making a 32 bit version way too slow to be playable, so they do this instead of releasing something that everyone would just swear at saying the devs are lazy/bad programmers. They can't just let them play the old game, as that would mean fracturing the player base, or letting some players compete in an environment with other players that get more stuff than them.
*I'm really simplifying a lot here, but the details are not really important for the discussion.