Skyrim had better diversity because of the technology. Oblivion had quite a lot of variation, it had what, 3 or 4 different biomes in the entire province? Quite nice.
About that many, but the problem was that the biomes were essentially painted on. You had forest, green hills, snowy mountains, not snowy mountains, yellow hills, swamp and maybe some yellow mountains. This was a bit of a letdown compared to morrowind, which had swampland, green mushroom forests, ashlands, lava flows, mountains, plains, rocky shore, mushroom shores and rocky wastes. Skyrim is more like Morrowind in it's diversity, and there is also more detail like Morrowind. This was largely a product of how Skyrim and Morrowind had hand made forests whilst Oblivion went for procedural.
Given that it used procedural the end result isn't bad, but it shows that procedural wasn't ready or wasn't used to its full potential, unfortunately, hence Morrowind and Skyrim have the better landscapes.