Making an original world means a lot more original stuff. You have to layout every castle, village, dungeon, and tree instead of copying a blueprint. Also more voice actors, lore, etc. They probably also chose it because it's a smaller project than making an original world(but still massive), and they love Morrowind and want people who haven't experienced it to do so, just with more modern graphics (I'm going to admit that I tried to play Morrowind saying "the graphics don't matter"...but it mattered and I couldn't stick it out more than a few hours.)