a major characteristic that makes it a perennial draw is imo that its so generic and mid. plus the technical side that it Just Works better than oblivion.

in a an actually good game completely altering core systems and questlines would actually risk you losing something---incidentally FNV has a lot less of this, and expansion-style is the main way stuff got added, since the original stuff is good. but with Skyrim? there's overhauls for every questline and mechanic, because the base was so barebones. ive played like 6 different experiences doing the Magic School in Skyrim with different mechanics & progressions.