Because most people are probably Hufflepuffs and Ravenclaws, and not Gryffindors or Slytherins. I think. I am not certain exactly where I would fit, or if even I would have been fit into the same category as I may be today.
I just love the conditional subjunctive verbs. If that's even right. Lucky we anglophones only have a few verb tenses, and not 11 like some languages.