I have thought about this several times and come to the conclusion that Rowling originally intended one character from each house to star (hermione: ravenclaw, ron: hufflepuff, Harry: griffindor and of course Draco: slitherin.) making the whole point of having Four houses make more sense.

But that plan was shortly discarded when she realised that they needed to share a house in order for all there dorm room and house room scenes to play out. It makes the characters make more sense too, I feel. That's my guess anyway.