Unless you plan on playing games on your apple tv, which tend to be rather large, there isn't really any need for it. I have had 32gb versions for both the 4th gen and the new 4k ones, and never had any issues with free space.
I think the reason it’s so popular is that it’s pretty cheap for an Apple device that has twice the memory of the lower model! Though as mentioned above, I think 64GB is massive overkill for the majority of people
Probably because 32GB is a lot more popular, so more of them get made. You generally don't need 64GB though unless you plan on keeping 50 huge games on it. Use it for what it's made for - streaming.