Morrowind mods tend to do things like fix bugs, design oversights in the skills (i.e. pickpocketing and enchanting), add additional, lore-enhancing content like GDR or Sotha Sil expanded, or update some of the graphics to modern standards such as advanced lighting and hi-res textures. However, even without mods, Morrowind is still an incredibly well developed game. Time can't tarnish the high-quality worldbuiding, ambitious art style, or the growth of your character from nobody to Nerevarine.
Skyrim, on the other hand, is a mash of poorly thought out ideas indicative of Bethesda's current dearth of talent. It's a mixed stew of fantasy cliché that never forms a greater whole. Dragons were added because Todd thought they were cool, but Bethesda failed to go beyond, or add an element of terror to the stereotypical creature we've all seen cut down by level one knights riding on horseback slaying monsters a thousand times before. Someone else (although it's probably Todd) thought that adding a civil war would be cool, but no one considered the consequences of adding this to the setting. As a result, we have the coziest civil war I've ever seen. Barely anyone seems to care that it's going on, and no one seems to be affected by it. Fixing Skyrim into a coherent, cohesive fantasy environment would require overhauling almost every faction questline. At this point, most modders would just switch to making a total conversion mod (Nehrim comes to mind, as Oblivion was similarly unfixible).
Morrowind was a game that formed a cohesive whole. The smugglers you fought at the beginning had names, good reasons to hide in caves smuggling goods as slavery and imperial bans and tariffs made it a lucrative career, higher organization through the Cammona Tong, and tied directly into the main plot through their smuggling of ash statues in the employ of Sixth House. The faction quests tie together with each other and the main quest, and the effects of house politics, though banned from open warfare, are felt all over Vvardenfell. This sets the stage for the the protagonists aforementioned journey from nobody to Nerevarine, where he goes from barely surviving against rats and wild animals to flying over the island slaying gods.
Skyrim is the exact opposite of this; you're told that you're the Dovahkiin after the second mission and begin with the ability to kill dragons with little effort. You go from a character who runs at a moderate pace, jumps a short height and slays nameless, generic bandits in 2-3 hits to a character who runs at a moderate pace, jumps a short height and slays the same nameless, generic bandits in 2-3 hits. The combat is as boring and it's as buggy as every other TES game but there's no lore, worldbuilding, or artistic creativity to make up for it.
I must admit that I can't tell what went wrong at Bethesda. It could be that the desire for profit stifled innovation in the interest of pumping out a generic open world RPG every two or three years with gimmicky release dates. On the other hand, I also suspect that since Oblivion Todd Howard has had a strong hand in using the games to vicariously live his personal fantasy of being a musclebound dimwit who solves all of his problems with murder.