books-and-shadows:

Slytherin won the house cup 6 years in a row before Harry started Hogwarts and then Gryffindor kept winning after that and I just find it really unrealistic that in 11 years Hufflepuff and Ravenclaw never won it once. If the teachers award house points just for answering questions correctly in class, it seems strange that the house that values intelligence and the house that values hard work above everything else never got enough points to win the cup at least once in all that time

Severus Snape was blatantly biased in favor of Slytherin house when giving and taking away points. We have seen that McGonagall, Sprout, and Flitwick were all relatively impartial in this regard, so the Slytherins had a massive advantage over the other houses for the six years before Harry came along. Of course after Harry arrived, Hogwarts lowkey fell to shambles and the following seven House Cup competitions had weird shit affecting their outcomes- time travel, international wizarding competitions, a literal takeover by Death Eaters, and so on. Additionally, we see point inflation occur over the course of the series- Harry, Ron, and Hermione dealing with the Sorcerer’s Stone earned them 50/60 points each while the very next year their tomfoolery with the Chamber of Secrets earned them 200 points each. The points are completely arbitrary. 

The House Cup competition hasn’t been fair for a long, long time, and that’s why it’s been dominated by Slytherin and Gryffindor.