Why doesn't Gryffindor's Quidditch win the prize every year in Harry Potter?

Harry didn't attend the first year of school because he was injured at that time. In the second year, he was suspended because of the basilisk incident. In the third year, he won the trophy. In the fourth year, Harry didn't participate, but Gryffindor won the trophy. In the sixth year, Harry still didn't compete, but he still got the trophy. In the seventh year, there was no Quidditch.