“That’s stickiness, and Rails has it in spades. Zero-second turnaround time; drop-dead simple and intuitive database access; simple, built-in testing; killer Ajax support; scaffolding; convention over configuration: those are the things that stick when you see them.”
I think that of his three factors, “stickiness” is the standout. Almost everyone who uses Rails for more than an hour or two wants to stay up all night and rewrite every web app they’ve ever written. I should know, I’ve already started doing just that.