People have too great an understanding of the game today for the simple (relative to today) raid mechanics of vanilla to be viable. Everyone would just blow through the bosses, or Blizzard would have to create some sort of artificial gear barrier to keep people from killing the bosses too quickly.
Look at it this way: A really experienced raider can look at the dungeon journal spell descriptions and instantly understand what the boss is going to call for from the player in that particular section. We all just know things about the way the game works that people didn't know in 2005.