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 लोकप्रिय as well. And, well. स्लैश just develops from there.