A bacteria has a lot of stuff that it needs to do in order to remain alive.
It tends to do that stuff differently from how our cells do it.
So if you stop that stuff from happening, the bacteria dies.

A virus on the other hand is arguably not even alive. A virus does so few things on it's own that many scientists seriously say that we shouldn't call that small chunk of genetic material wrapped in protein "life". It is very hard to kill something that isn't even alive.

The medicine we do have concentrates on sticking to the virus or the sticking to the places the virus sticks to so it can't get in.
Targeting the stuff that makes copies of it self doesn't work well because our cells need that to remain alive.