Imagine if like in the second year a kindhearted slytherin joined the group (Harry, Hermione, Ron) Wouldn't that be awesome! It would have showed the readers that not all slytherins are bad. It would be awesome for them to have a friend from slytherins because through their first year the entire time they hated Slytherins, And for them to actually get one as a friend would change the whole dynamic!