Bethesda owes everything to the PC market, and they know their remaster really doesn't offer anything to those PC users who have bought all of the DLC. As everyone else has said, the available mods will likely outperform anything they're releasing with this remaster, or at the very least will match it.
I think they just looked at their PC market, recognized it as being one they owe their success to but also one they cannot hope to sell many of these games to, and decided to just gift it to them as a nice gesture.
*Edit* For the record, this strikes me as a waste of time, resources, and manpower. If they were going to remaster a game it ought to have been Morrowind; given how popular they've become since then I imagine that game would sell like crazy.
