He doesn't have any tragic, bloody story that I remember, or rivalries with anyone, and seemed quite welcoming when the new students showed up. So he's probably the most well-balanced and helpful ghost, at least, which is a point in Hufflepuff's favor. And it would work nicely if Hufflepuff is the most Muggleborn-friendly. (I could see it going either way, since if directed a certain way they might end up super-loyal to the Pureblood cause or something equally unfortunate - but that doesn't seem to have happened. Foolish Tom didn't bother recruiting there, probably, even though he might have managed when he was handsome and charming. And we never seen Justin take any flack from his fellow Hufflepuffs. So I'll sign on to Terri's theory.)