I'm not sure exactly, but I think it's mainly because it does away with the overused blog-headline cliche of the "Top X" list. It puts focus back on the content rather than the structure, and makes the headline less obtrusive when listed with all the others.

It just seems less hype-oriented and attention-mongering, similar to how (if I remember correctly) HN automatically removes exclamation points from link titles.

[EDIT: Or maybe I just like that it's quirky and contributes to the particular "voice" of HN, like how the New Yorker's quirky styles for diaereses and acronyms contribute to its particular voice.]