Hrm, good question.

Spoiler:
Remember that Snape was in Slytherin as well, and he is a half-blood, as revealed in the Half Blood Prince. His parents were Eileen Prince (a witch who had previously been in Slytherin) and Tobias Snape, a Muggle who wasn't too kind to his wife...


Maybe it's just because Salazaar Slytherin was a little more conservative and less acepting than modern wizards. That would be my guess.

P.S: Saw the film today; absolutely excellent. The original elements, such as the beginning, and the horrifying event at Christmas, almost made up for the omissions.