Well, it's probably not because of quality! I tend to shove my pics in as many groups as I can find, so most of them punch above their weight in terms of favourites.
It's a weird yet predictable place, deviantART. I've seen pieces of work which are blindingly good, easily worthy of being put up for sale at a good price, which have had virtually no interest. Yet as soon as some cosplay girl gets her arse out, it'll get six gazillion favourites.