Just a different evolutionary path. As someone who loves all the diversity in manga, it's really a shame. In Japan, manga is directed at many different demographics. It hits so many genres that almost anyone can find something that corresponds to their tastes.
I kind of wonder if the biggest difference was that Western comic books were never able to appeal to girls in great numbers. Shoujo manga goes back a long ways and it began to really gain popularity in the late 1960s. This provided a counterbalance to shounen manga and it's not rare to see them take elements from one another.
As for animation, it hasn't been that long since it carried a pronounced stigma in the US. I remember a friend of mine once saying something like, "animation is just for little kids," and it really, really bothered me because I'd always loved the unlimited potential of animation as a medium.
