What's so good about the game that keeps players coming back even with Skyrim?
Levitation probably.
Honestly, hard to say. It's just a really well made game with lots of little details and a fully fleshed out world that's fun to explore. Everyone's got their reasons, I guess. I also second the person above, the combat is pretty dumb starting out. I just think the whole RNG chance to hit mechanic doesn't really translate well in first person since everything is in your face and happening in real time. It just looks bad. In a game like Neverwinter Nights, it's a bit easier to stomach since you're further back. Some appreciate this, because this makes it more of an RPG, since you actually have to be good with swords, and not just good at controlling a character and making them hit stuff with swords. Character skill vs. player skill and all that jazz.
I think Morrowind was one of the last GOOD RPG-centric open world games released. (We can more or less forget about Two Worlds...) The later games moved away from that, and there isn't much else out there that offers such an experience. Oblivion and Skyrim are good in their own right (the combat HAS gotten better) but for those looking for a more hardcoe RPG experience, it's not really there.