As someone one in the US, I think part of it was the lack of advertising. Most shonens tend to grow big due to having an anime. As a teen most of the mangas in Shonen Jump had animes running on TV, which made it fun to read and watch together. However, most shojo animes were not given the same treatment. I think this might be why some men mention growing up with Sailor Moon, it was one of the few shojos that played on TV, with the only other one I can think of being Card Captors.