Just my opinion, but it's probably because of the way regional mythology evolved and spread, and that many asian dramas include creatures more popular in their specific countries, histories, cultures, etc. Werewolves and Vampires, as they're specifically represented in modern pop culture are more based in eastern european or greek or roman or norse mythology, so it's much more likely to have spread to America.
As things get bigger in pop culture they tend to spread further though. There are quite a few asian dramas that involve stereotypical european vampires in the plot, but wayyyy less than the thousands upon thousands of english titles. Likewise, there are probably many other myths or creatures that show up in asian media that show up much less in north american media, if maybe only as a reference and not a part of the plot. Sometimes once a myth is featured for the first time in mainstream media it becomes more well know, and then you'll see 10 more movies pop up about it in the same decade.
There are also times when similar creatures that actually originate and involved in different regions across different eras and evolve and spread in various ways, and this tends to expedite the process. As a very basic example "Dragons" are ubiquitous across the globe as a general concept.
