What's the in-universe explanation for why more vampires in the Buffyverse don't use guns?
Darla does in the first season and here and there, we see vampires use guns. But most seem to eschew them. Why?
There's no real in-universe reason and, as you mention, Vampires do seem to occasionally use them.
That said, guns require background checks. If you are dead that's probably going to be a pretty big flag to stop a gun sale.
Also, Vampires can pretty much rip humans in half - strength wise - they really don't need them.