Mainstreaming for people who couldn't handle the rulesets of RPGs. Let's not forget Morrowind was Bethesda's first major success all around on PC.
Hell it was right behind Halo on the charts in the Xbox days.
You can't say between Wasteland, Planescape: Torment, and Daggerfall to Morrowind there's a vast plain of loss in RPG mechanics.
Morrowind to Oblivion was less profound a step. BGS took a new direction just like Morrowind. The difference was Gamebryo wasn't as great, they weren't as prepared for the 360 due to late specs and the PC version suffered, and all around problems in script and writing both due to staff like Emil plus the major costs of fully voiced characters. Even if it was like 13 people.
The TES series as a whole isn't like a 2nd edition D&D set. But Oblivion wasn't any road to being a mainstream RPG, Morrowind was.
And PC or Console developed other than DLC it was inevitable.
The gaming market would of grown to current standards anyway. And with so many core fans of different genres of action, adventure, exploring, Fps, tps, tbs, etc. it would drop complexity.
Not that all is lost. From the mid-2000s where linearity ruled we're seeing freedom and old mechanics as new selling points. One day too Daggerfall mechanics revamped will be new selling points.