Drugs kill germs by messing with their biological systems. They target specific processes, like preventing enzyme from properly bonding so that it fails to do something important in the reproductive cycle or whatever. If a new generation of bacteria evolve such that that specific process works differently, it could kill the effectiveness of the drug. And that's what happens when something becomes resistant to a certain drug. Suddenly the aforementioned enzyme and the reproductive cycle are ever so slightly different, and as a result the drug can't do what it used to do, at least not as effectively.

But UV just straight up breaks up the bonds between molecules. There's nothing biological about it, its destruciveness is entirely physical. The photons get in there and start destroying molecules, living or not. It's not easy or likely at all for a strain of bacteria to randomly evolve resistance against physical destruction at a molecular level. They're generally too small to have a protective layer to shield them against that, like our skin does.