Replace “vampires” with “urban fantasy” and the statement still applies. This isn’t just a problem with vampires but with urban fantasy in general, and basically with any non-Tolkienesque/D&D-esque fantasy.
Yes, but unlike most other urban fantasy fare vampires have received quite a few games over the years.
You had Bloodlines, obviously, then there's Dark, Bloodrayne games (specifically 2), Vampyr (sorta counts), Blade 1 and 2, some Castlevania games far as I can recall - and these are specifically games in a modern(ish) setting where you are playing as a vampire.
And if you look at all the games I've listed below, and then try to find a similar lineup of games focusing on other fantasy critters you'll have to admit that vampires feature far more prominently than other monsters, not just in games, but in other media as well - just look at shitload of movies, books, comics etc. focusing on vampires.
How many medieval fantasy games prominently feature vampires as a playable option?
Vampire: The Masquerade – Redemption, Castlevania games, Bloodstained, the Legacy of Kain games (though, only Blood Omen 1 and 2 specifically focus on the protagonist being a vampire), a Vampyre Story.
How many games feature vampire hunting? I can’t think of anything besides Wrath of Malachi from the early 2000s.
Darkwatch, that Hunter game from the early 00s, the Buffy game adaptation, Bloodrayne 1 and 2, Nocturne, the Witcher games, the Dawnguard DLC for Skyrim, the Incredible Van Helsing, the Blade tie-in games, Castlevania, Bloodnet, Vampire Rain, Resident Evil 8, Order 1866, and I'm likely forgetting a bunch more.
This seems to be an issue with video games in general, not just crpgs.
My argument really has nothing to do with your weird tangent about developers not bothering with specific narrative niches. There's clearly been an interest in making a (spiritual) sequel to Bloodlines, as evidenced by the existence of games such as Dark, Vampyr and Bloodlines 2, and vampires still retain mainstream interest - yet all these games have failed to live up to Bloodlines' legacy specifically because they kept trying to reinvent the wheels.
So my point still stands - if there is an interest in making a vampire-themed game that taps into the Bloodlines audience, why has nobody been able to successfully copy it?