Because different people have different tastes :$
I really liked Fallout 3, but I found Oblivion to be a bit tedious (albeit fun in small doses)
I think the reasons why Oblivion didn't grab me were:
a) I like a plot I can sink my teeth into
b) Lacklustre characterisation of npcs. Coupled with poor voice acting and strange faces I found it difficult to identify with any of them. I think the only one I can remember now is Martin.
c) A 'standard' fantasy setting that boiled down to lots of bland wooden towns with forests inbetween. I read/watch/play a lot of fantasy and am pretty bored of that setting by now.
d) Little variety in the dungeons. Hello generic cave #20, Hi Oblivion gate #15!
e) A combat system that basically comes down to clicking the mouse a lot, with very little feedback on whether you've connected. I like my fighting to either be more visceral, or more tactical
f) A messed up item levelling system. You spend hours questing to get an awesome sword and armour. Then you level up and suddenly every single bandit gets your ideal loadout. It spoils the fun of questing for something when it becomes so common, so quickly.
On the plus side, I thought the attention to detail was fantastic in regards to the countryside, the ability to enchant items and craft your own spells is great, the world is huuuuuge, the general freedom is fun, and The Shivering Isles fixed some of the problems I had with the main game (interesting location?thankyouverymuch). Oh, and I thought that the 'living world' thing they had going on was a nice idea - the first time I saw someone chased and executed by the guards for stealing was impressive, even though I did sort of want them to train me in something...
So all in all, it was a fun game, but not one of my favourites. And there's a rundown of some of the reasons why. I can see why people like The Elder Scrolls so much, but it's just not my cup of tea. I'll probably get and play Skyrim, but not until next year sometime (or maybe later).