As we start the series we see Harry's prejudice for Slytherin house and I think it was the perfect thing to give Professor Snape a redemption arc in order to break the prejudice of Harry and others against Slytherin house.

Slytherin house was always considered as related to Dark Magic and to cope up with this Slytherin students had assumed that they are superior and need nobody else.

The removal of stigma against them ​in return would have resulted in more inclusion of Slytherin students as they were not been seen as different.