Surely this is a really simple reason? In those 200 years everything has gone wrong for the Empire, now there may be a few here who would enjoy playing a game where you essentially lose it all at the end, but for the average gamer they want to be a hero, they want to win. There is no role the player could take on in those 200 years where they could be a hero, nothing good happened for the Empire, so they skipped it and put most of it in dialogue and books in the game as references/lore. It also sets itself up nicely for more heroic adventures in taking the fight to the Dominion, liberating lands and rebuilding the Empire.