Not the only reason: It offers a stable library, consistently available, at very decent download rates; as far as I’m concerned that’s worth the nominal fee. Filesharing is a series of hoops through which it is at least annoying to jump, and lately somewhat dangerous. I’d bet my hat that convenience - and some thin veneer of possible legitimacy - is the cause of AllOfMP3’s success.
I suppose that’s a hijack.
You could always subscribe to Napster or something - listen all you want to anything - and buy whatever you liked. That’s what I’d do if I had any disposable income at all.