For people with access to mods, there's no need. Think about it, you can achieve Skyrim(ish) graphics with Morrowind, fix the levelling issues in Oblivion, and add in anything you can think of. I could happily play Morrowind and no other TES games. That being said, they're my favourite series of games.
There's also the opinion that TES games are going downhill, which could contribute somewhat. (Lets not start a flame war).
Hm... well I find that modded Oblivion still feels like Oblivion at heart. Wouldn't you want to try the whole new levelling system (I don't think there are any mods which change the basic skills and adds a perk system), more handcrafted game world, new quests and such with Skyrim? I mean if you like any TES game enough to spend time on these forums, surely more of it is still good.
And yes, I know many people thought Oblivion was really bad compared to Morrowind, and while I can't really get into Morrowind for some reason, I agree that many features have been done better in it. But from what we've heard, I think Skyrim is fixing a lot of issues in OB, notably level scaling, small settlement number, AI issues, travel system, mundane feel, etc.. I have basically no doubt that it will be at least somewhat better.