I mean they aren't instant and have to be within a fairly short distance of the thing you want to sterilize in order to work because they are absorbed by the air. Something like a pool would be practically impossible as water also absorbs UV and a pool is too big to penetrate all the way through just from the sides or bottom. It only works for drinking water because you pass said water through a tube that must be fairly narrow.

Oh yeah and an X-ray could sterilize all the way through an object, not just the surface. Very useful for making things like microwave meals.