Well, it wasn't. Oblivion was a huge step down from Morrowind, in terms of hand-holding, the sense of being in an alien world, and variety of landscape/culture. Skyrim maintained much of this hand-holding but there was more variety in culture and landscape (Oblivion was basically just one province, very same-ish, aside from the Oblivion gates, which were not very well done) . Both graphics and combat were improved in Skyrim.
Expectations and experience with the ES series play a large role in this, I think. Oblivion was the first ES game for a lot of people, and so to them, it's the standard bearer, and they see it through rose-colored glasses, imo. If you had played Morrowind before it, though, you were as a rule disappointed with Oblivion -- found it extraordinarily dumbed down and lacking in a lot of what made Morrowind special. By the time Skyrim came out, though, you were past that, and your expectations for ES were reset -- you knew Skyrim was going to be a mainstream game, and you were okay with that; you just wanted it to be better than Oblivion. And it was.
Morrowind > Skyrim > Oblivion.