Could it be we are only getting an OVA because GOHANDS are still working on the 2nd and 3rd movies of Mardock Scramble? If they aren't that big a studio it might spread them too think or cause too much outsourcing to work on another Seitokai series at the same time as the movies.Because in terms of dvd/br sales Seitokai has been their most popular production easily and should definately have been fairly profitable.
It's not that the sales weren't enough, it's that they don't have enough content. The anime covered three volumes of the manga, and there are only four volumes currently out. So, rather than more seasons, we can probably expect OADs to be included with manga volumes, as is the trend recently well recieved anime adaptations that don't have enough content for entire seasons, like Black Lagoon, HOTD, Yozakura Quartet, etc. Although Yozakura didn't sale that well.
@Westlo/Suzuku: I see. So there were only 4 volume of original manga. Thank you for more detailed information. :)