Pretty much what desi said. These titles are cheap and quick to translate, and they're guaranteed to bring profit.
Additionally, I'd like to note that it was "that" dreadful AX panel that caused so much not entirely warranted discontent about MG. People expected at least some story-driven titles to be announced, but there were none. It was a really bad move for MG, but in all honesty, it's not like the ratio of cheap nukige to story-driven titles changed that much. MangaGamer always has at least 2 and as much as 4-5 story-driven VNs in the works pretty much against business sense, as those titles can bomb spectacularly (Kara no Shoujo, DC fandisks) and rarely become a huge sales success (Shuffle!).
The argument regarding time and effort that could be spent working on story-driven titles is somewhat solid, but it's really not fair to say that releases of story-driven titles slowed considerably because of that. It is not MG's fault that Harukoi takes forever, cause the work that stalled the project is carried out by a third party.