Because after a rental, I deemed that, while they've made some improvements since Oblivion (equipment degradation, stupid speech minigame, that godawful leveling system), the core problems that made me dislike the series are still there.
Level-scaling: Level up the wrong skill-set in the wrong ways, reach a point where one bear will rip your face off without a problem.
Lack of flow: The sidequests seem to follow a philosophy of "throw it all at you and see what sticks," giving no indication of relative difficulty nor reward, and a difficulty curve that was apparently inspired by the output of the dev computers' PRNG (and is unrelated to the reward value).
Unskippable voice acting: Yes, I get it. Your brother is missing, family feud, blah blah blah. Shut up and let me get on with it. I just heard all this crap five minutes ago before the game crashed/on my last character.
The only slightly less godawful leveling system: See level-scaling and for the love of the gods, don't grind lockpicking early!
Maybe in three or four years when the inevitable GOTY/Platinum/GH edition is available for $10 on GoG, I'll pick up a copy.