Oblivion worked, and despite its issues it was playable. It was also more accessible, especially for people new to RPGs, and I think it had that in its favour. Morrowind didn't have that accessibility or that ease of use. Combat was beyond terrible, you were kicked out into that first village with no real sense of direction, and that was it. It didn't guide you through a small area (Like many, many RPGs do) to get your bearings, and it was just a very cumbersome game.
I'm pretty sure a significant amount of fans that would agree that Morrowind worked just fine for them.
Being less "accessible", and not holding your hand, were actually some of the reasons many people preferred Morrowind.