Camera can't have 4k and 60 fps?

I have a droid turbo 2 and it has the option to do either 720p 120 fps, 1080p 60, or 4k 30. I got a third party app that allowed me to set custom resolution/framerates but when I set it to 1440p 60 it said "not supported". Why can't I do that or even 4k 60? Is it a storage bottle neck or cpu maybe? Thanks