Because as everyone knows, if you win a battle at Coruscant, you automatically gain a huge military out of nowhere.
The amount of good lore dying by the second is sending enough of a shock through the Force that George Lucas is thinking of getting involved.
Yeah, we haven't covered the full 1,000 years in the TL yet (never will either, at least not ICly). But the Sith sure took a gigantic, and somewhat unreasonable (imho), leap forward.