Regarding vampires, my go-to theory to explain why they were not a big thing beforehand: they used to be nearly extinct. The Barrier the Sorcerer Supreme maintained, combined with centuries of effective action by various monster hunters and mystics and such, had effectively purged their numbers such that, come the 20th century? Vampires basically only still existed in one or two isolated stronghold locations ( think "Transylvania" ), and maybe a double handful of wandering predators *worldwide*. Groups like SHIELD or Hydra may or may not have been aware of them, but only as lone oddities of no real importance, and Kamar Taj knew about them, but only as one of many low priority occult hazards. Thus, the only people to really care about vampires were the odd vampire hunting lineages that might still remain, and most of them are focused on hunting *a specific* vampire, more than a species.
This all goes out the window after the events of Dr Strange 1. The Barrier dropped for a while, a big wave of dark magic flooded in, and suddenly a lot of the old monstery things and easy black magic that used to be dormant is now active again. This includes vampires: the active ones are now much more powerful, a whole lot of slumbering vampires just woke up, and everyone finds it much easier to embrace new fledglings than before.
