Well, I think the issue is a combination of a lot of factors. One of them is that a lot of the more "traditional" monsters don't work that well in a modern sense. Things like a Kappa are scary when the world around you is largely unexplored, and walking a dozen miles through a forest or having to camp overnight is an epic journey. When there really aren't any dark and unknown places anymore, or at least not like there once was, the truely monsterous falls out of the popular imagination. There has to be some reason why these creatures are not commonly known or seen. Ghosts, Vampires, and even Lovecraftian horrors have reasons for remaining unknown/blending in. On top of this I think there is an issue akin to what you see with Hinduism that a lot of these monsters have religious, or at least spiritual overtones, and there are lots of people who take them seriously as part of their faith. The final issue is of course shame, while you could argue there are vampire-like elements in Japanese fantasy going back to the 1930s, I don't think we really saw anything on a large scale until Japan more or less put a bullet in the head of most of it's own culture and started to embrace and modify Western culture to replace their own. Their take on sword and sorcery, horror, monsters, science fiction, and other things is all very Western, that's one of the reasons why things like Anime and Manga have traslated so well. Vampire are hot in the US, and have been for decades now, so as a result they became hot in Japan. It's also important to note that a good portion of "authentic" Japanese history and cultural trappings are also fake. A good example of this are Ninjas. Ninjas as they exist now are a western invention, being far differant from their historical counterpart. They used to be well hated, but Americans heard about it, became inspired, re-did the concept to be heroic or anti-heroic given a differant idealogy, Japan resisted at first (think back to some of the Japanese responses during 1980s "Ninja Mania") but then decided they liked the idea, and started running with things along tha direction to the point of actually trying to present it as having been that way at times. The same could be said for the Samurai to a lesser extent, I remember reading a big thing years ago about how the American Western changed Japanese history since the current image of Samurai or Ronin now matches the gunslingers of that genere very closely. The same Aristoricy which was overthrown by peasants in a great revolt, is now a source of nostolgia and national pride. It's really quite wierd.
Xenophobia has always been an issue with Japan due to their entire "master race" doctrine. Having been beaten constantly by the biggest group of mongrels on the planet (the USA) they have developed a sort of love/hate relationship with us that is truely bizzare, and of course the culture has started to change under great resistance. It's disturbing when you see popular culture that is pretty much pining for the old days, or talking about 'setting things right' by having The Japanese take control of everything and there being almost zero people not of that ethnicity on the planet anymore. I can see how the Vampire can fit in with that attitude, and might have been a propaganda tool originally, but today I think it's largely a matter of it having become cool there, because it's cool here.
This is what I think from having read quite a bit about the subject over a number of years. The understanding I've come to has a lot to do with why I never became a Weeaboo or anything despite consuming a decent amount of Japanese pop culture.
When it comes to foreign occupation, that's touchy for a lot of reasons. Japan is very much a nation under US occupation. It occupies a truely wierd place somewhere between puppet nation, ally, rival, and conquered territory. To put it bluntly Japan is our major foothold into the Eastern World and where the USA bases a LOT of it's ships. As a result we're never going to leave that area no matter what anyone says or does, our military forces there greatly overpowering things like the SSDF which we only allow to exist as a diplomatic gesture. It's hard to entirely resent this though, due to the massive cultural changes that have a foothold, and the simple fact that we're also Japan's bodyguards. Japan still exists because the USA protects it and has huge amounts of military power there. This is what allows Japan to deal with other nations diplomatically. A lot of nations around it like China and Korea have been horribly victimized by the Japanese and would love to pretty much wipe the place out. We leave and the diplomacy you see now ends, and Japan dies. I don't think anyone has many illusions about this, especially when you look at some of the pop culture from places like Korea and the seething hatred it has toward Japan. I read parts of this Manga called "Island" once that had sections in the back (including photographs) talking about the historical events that inspired it, and the experiments Japan was doing on Koreans. "Unit 731" was not their only group up to this kind of thing. People know about their uber-rape campaigns in China, but really that's only the tip of the icerberg and people just aren't that forgiving. To Japan it hurts their entire sense of cultural/ethnic destiny when they
are in a position where they pretty much need foreign occupation in order to survive. Hence the mixed messages you see, combined with some of the more over the top power fantasies.
