It is a very, very casual game that came out at the perfect time and caught lightning in a bottle.
The game is trash in terms of design (bad dialogue, bad combat, bad dungeons and bad quests. but the production values and marketing made that shortcoming less of an issue; Skyrim did get exploration very right, I'll give them credit on that front (although that is mostly due to the great map).
Same thing happened with Fallout 3: lots of fear of Oblivion with guns and that is exactly what happened. Crap game that doesn't give any satisfying rewards but because people like to talk about how immersed they felt rather than actual gameplay, it didn't really matter: people prefer walking straight for hours with nothing to do if it immersive. Well designed maps with no downtime get hated on for not being realistic.
Popularity is absolutely meaningless when discussing quality of design.