why is it so hard to up strum with your thumb on guitar?

It's hard to strum with the thumb. Typically in fingerpicking, the thumb is the bass player. In the realm of fingerstyle, the thumb is used quite a bit more. 

Restricting hand movement. 

because of fear

Probably because you're not used to doing it, or you're doing it wrong.

I think usually it's not just the thumb that's doing the strumming, it's the thumb and the first finger that are in the same position as they'd  if a pick was being used. The nail of the finger strums going down, and the nail of the thumb strums going up - that's how a ukulele is played. It's not a comfortable thing to do until you get used to it.

Thumbs are made for gripping and clenching, which in guitar orientation is a downward motion.