Seeing that nobody answered this question, and I took some time to look it up. Here is the answer I could find for the people who also have this question.

The reasons that every girl is in Obelisk blue is,
There aren't that many girls applying to the school, so it would be very costly to have different dorms for a handfull of girls. They even lowered the passing grade to enter the school specially for girls, just because there are so few.
Now, you might ask, so why Obelisk and not Ra or Slifer, Obelisk is for the rich kids?! Well, I couldn't find a logic answer for it, but the best I could find was, just because the Bleu dorm has the highest quality of living, and they decided that girls deserved the highest quality. 
But I don't find it logic because that would be sexist and very contradictory. You mean to tell me that to save cost you put all the girls in one dorm, but not just any dorm, the most expensive one. Doesn't make any sense to me.

But those are the best answers I could find.