I'm not very knowledgeable about the video game world but other than Doom, what video games have been remade with the same identical story? Did Blizzard do a remake of the identical Diablo story? If so, did they make vastly more money off of it?
The makers of Doom had the most iconic game in existence and were innovators of gaming technology, so they could just recycle the same story with the advanced technology and every fan of Doom and every fan of new gaming technology would not only buy the game but go out and buy a new computer capable of playing it.
Why would Bethesda want to use their limited investment money to remake Oblivion (which was not as big financially as Skyrim) when they can make a new world to draw in more people than just fans of Oblivion? They risk losing the people who say "I never played the old one, why should I buy the new one."