I find that a number of my friends only play D&D because they don't want to process a new set of rules. They "know" D&D. They like other settings and genres but not other rule-sets.
D&D is the one I know the best, but it's not the one I like the best. I've fiddled with a variety of RPGs now and D&D is now high on my list for disliked RPGs - mostly for the level element and that it is not a skill-based system.
I'd still play it if I was asked, but right now Star Wars D6, World of Darkness and especially Savage Worlds rank far higher on my go-to list. And having just read FATE, I suspect I'd like that system a lot more than D&D.
As I have gotten older, my interest in learning new game rules has waned. Primarily because my free time has diminished greatly.