I can't claim this would be true of everyone, but I tend not to consider art when requesting because it's above and beyond what I think to ask for rather than because it's less than what I ask for and so a disappointment.

I'd be over the moon to receive art in one of my pet fandoms, and I'd wager that's true of at least some of the folks who only mention story details. They may not want to build up their hopes in vain or might not think they could get something so pleasing and rare.