In some of the original myths, the best way to make sure you became a vampire after you died, was to be a werewolf during your life.
Also vampires would occasionally turn into wolves (as well as bats or mist) so yeah the werewolf vs. vampire thing is recent since they were sort of intertwined before.
I imagine the main blame is probably just Hollywood, going all the way back to when they were starting taking the classic movie monsters and mashing them up in the same movies like “Dracula vs. Frankenstein.” I think the first written example I ever read was actually in an old Mad magazine. And I mean the original Mad Magazine from the 50s that resembled more of a comic book.
There was a parody story about this guy going out with some hot chick and barely escapes after he finds out she’s a vampire. He stakes her during the day (I think) and goes on talking to the readers saying it was a shame she attacked him since he really wanted to show her something special. Then the full moon comes out where upon he turns into a werewolf and says something about its just as well he killed her because he didn’t need the competition from vampires.
You can have all sorts of monsters in “vs.” scenarios, but if you take all the “classic” movie monsters, you can really only do the vampires vs. werewolves on any large level since they can usually fit into human society in some way, will have territory competition overlap and have all sorts of secret wars and shit.
Mummies tend to be singular. These were kings and queens so you aren’t really going to have armies of them (Nope, not even Warhammer’s Khermi had hordes of mummies) to face off against packs of werewolves and broods of vampires. Also they’re a little more obvious with the whole walking corpse thing. Yeah I know they’ve updated them a bit so they look human, but again that’s a recent invention.
Frankenstein (Or “Monster” if you’re going to be technical about it). This was a flesh golem. And again they don’t usually have hordes of those mainly because they’re a pain in the ass to make. Unless some mad scientist is REALLY cranking them out on some assembly line, again you’re only going to have one of them. And again, they’re obviously not human looking so they would be more likely to be targeted by angry humans with torches immediately rather than a vamp or were having to bother.
Same goes for the clay version, though exchange mad scientist for Jew magic instead.
Creature From the Black Lagoon (Or “Gillpeople” I guess). Now you could make the argument that there might be a whole ocean full of these fuckers which means you could have them fighting packs of werewolves or vampire broods, but unlike weres and vamps, there probably isn’t any real competition overlap. Weres and vamps fighting makes more sense since their territory overlaps a bit more.
I mean if there was some vamp/were living on the coasts, I suppose they might be more of a problem, but again they’re so obviously not human looking, if an army of these came marching out of the oceans, humans would be directly attacking them and taking care of the problem.