WHY????????????????

Because it started out as Severus and Harry having a rather high dislike of each other, and then we discovered that Snape was trying to PROTECT Harry the entire time, and they sort of reconciled somewhat, and then Snape DIED

Plus, Severitus (That is, Snape takes Harry as a apprentice, a student, adopts him, etc). was quite popular as well. And, well. slash just develops from there.