@Cyriel Such a move would be great for some people. But the international exposure of the doujin community would also bring with it a whole slew of problems that Steam wouldn't be willing to help with, instead leaving developers in the dust.
For now, DLsite is still very necessary to provide a "safe haven" for small developers and hentai content. Until people learn to relax more about erotic products being offered on the same marketplace as mainstream entertainment, I don't think some of the groups publishing on DLsite would even want to join Steam.