Here's why I roleplay, and why I now prefer the older, simpler kinds of D&D over the new, slickly produced versions (my ideas about why I prefer D&D over other RPGs will have to wait).
1. I like to have fun with my friends when I game. I'm not looking to create some amazing story, or some amazingly in depth character, or anything like that. Just have some fun.
2. Older versions of D&D, especially my edition of choice, BECM, are simple games to prepare. In an hour or less I can whip up a 12-15 room dungeon, with monsters, traps, and treasures. And it only takes me an hour because I'm a little rusty with the rules. With 3rd Edition, that one hour would likely have been spent on one or two rooms only (haven't tried DMing 4th, only gave it one play-test as a player so far).
3. In my experience, those older, rules light game systems tend to create more and better roleplaying. If every Fighter is mechanically the same except for Ability scores and magic loot gained, then it's up to the players to roleplay to make the two Fighters distinct. With more 'skillz & powerz' type systems, I've noticed that people tend to distinguish characters by what they do in game, mechanic-wise, rather than by personality.
4. I'm a father and husband, and am currently working 1 full time and 3 part time jobs. I don't have time to mess with rules heavy systems with a ton of splat books coming out all the time. Gimme some short, simple rules that I can tweak to my liking, and I'm good to go.
5. I've got nothing against newer games if I'm a player, but I almost always end up the DM, so I'll stick to the old TSR stuff, with my house rules.