I wonder if it has anything to do with the series still being ongoing?

SPN has a ton of longfic and it's still going.

I think it just has to do with the nature of that particular fandom. I also think it has to do with the size. TVD is not a tiny fandom by any stretch, but when compared to the likes of BtVS, SPN, Sherlock, and Harry Potter, it pales in comparison. The more people you have writing, the more types of fic you'll have. The one shots outnumber the epics in all of those fandoms, but there's enough fic written that there are plenty of epics to go around.