It's a vicious cycle. Comic book stores sell products that appeal to adolescent men, because their customers are adolescent men, because comic book stores sell products that appeal to adolescent men.
You can easily find English-language translations of the kind of Manga you describe, or of Franco-Belgian comics, but most American comic book stores won't make any effort to stock them, because they don't expect to sell them.
Here in Europe the comics market is more diverse, but that's as much an accident of history as anything else. For "normal" comic shops to exist, you need both a tradition of "normal" comics and a market for them, neither of which really exists in the US. You had Stan Lee and Siegel & Shuster, we had Hergé, Franquin and Peyo, and from there we diverged.