I don't have a quote with me at work, but I know the quidditchQuidditch wins are a very heavy weight in your house's total points, I believe itsit's close to 50 (we actually dontdon't know how many points, we just have a scene with Harry geting docked 50 points thinking that it will looselose all the points he just won the house from winning a quidditchQuidditch match) points a win, with a bigger win for the cup at the end of the year. Since the Slytherins had been winning quidditchQuidditch a lot before Harry, combined with Snape's favoritism to his house, this is most likely why they won ever year.
fixed grammar, spelling, punctuation