As far as i know for many people in japan becoming a manga/anime-artist is a dream come true, there are sometimes even too much aspiring artists, so they work on tight hours and for people they don´t like to work. Hell, there are even a few mangas about becoming a manga artist. Good animations are expensive, that´s why many artists have to settle with hentai instead of anime to get around and maybe even become famous. If you search through the artist databases there are quiet a few people who did hentai before getting into a team for now popular animationseries. If i recall correclty even Naruto, the most mainstream anime to date has quiet a few ex-hentai artists. Some Animes even start with a hentai-pilot episode like Mezzo Forte.
http://goboiano.com/29-famous-manga-and-anime-artists-that-have-done-hentai/
edit: tl;dr: Hentai sells better, that´s why there are more studios for it and it´s a better (or the only) starting point for people who want to get into serious studios.
This is only the information i gathered, it does not have to be the fact, i could be mistaken after all.