Probably cause sitting on disasters makes it look like you're hesitant/scared of going back in, while with rock n rolls it looks more like you're comfortable with the trick.
I know that when I was learning disasters, I definitely camped on them for awhile, before leaning forward into the transition, cause I was afraid of slamming.
It's kind of the same reason why I don't like crooked grinds to fakie, cause that's how I first did them, just falling off the ledge.
I'm not really old or grizzled though.