thats a good question!
i have a book called "Image Factory" that deals with this phenomenon as well as others.
in general, i think it stems from highly structured Japanese society and all the Kawaii-ness as an outlet for that. sort of like being able to still have that bit of childish freedom.
As for why americans are so engrossed, i really dont know, but i an definitely a victim :)
i think maybe americajin regard japan as more technologically advanced and figure whatever is cool there must be the "next big thing" and want to get in on it.
*shrugs*
it works the other way though. nihonjin are just as crazy about american culture.
good topic!