Why is Skyrim getting an HD overhaul and not Morrowind?

Answer 1

Basically because it would take as much work to remaster Morrowind or Oblivion as it would take to make a whole new game. Here is Pete Hines (a marketing VP at Bethesda) speaking on the subject in an interview with finder.com.au:

"An Oblivion remaster is an interesting idea, but it is ten-year-old game, and Morrowind is 14." ... "You are talking about several engines ago." ... "So the question becomes, 'should we work on a Morrowind or Oblivion remaster, or should we work on Elder Scrolls VI?'" ... "We have to ask "do we want to take time and resources, and spend money, on something that is already out to bring it to a new platform, or do we want to forge ahead with new stuff?" And in most cases, the answer is that we want to forge ahead with new stuff."

The Elder Scrolls V: Skyrim Remastered should be out on PC, PS4, and Xbox by the end of 2016. Not all of the old mods will work for the new game. Some will have to be reworked.