Mmmm, I can only say that people tend to gravitate toward the unusual out of curiosity. If you grow up your whole life surrounded by blondes, then naturally you'll be intrigued by a dark-haired girl, provided that she looks healthy.
I would say that I'm one of those "yellow fever" gaijins that are often whispered about... but whatever, I like what I like, and anyone who wants to judge me I say to you.... SCREW YOU. It's none of your damn business anyway. <ahem> rant over.
However I'd like to point out that the original poster is only referring to Tokyo. Try traveling out into the country sometime, like Gunma or Yamanashi ken. You WON'T be saying that all asians are hot after that.... trust me. Tokyoites and other urban dwellers take better care of their appearances than their rural counterparts. I think this applies worldwide.