I think it was a plot point that the Legions were just barely holding together after the Oblivion crisis? It's not like the Redguards can come instantly, if the Aldmeri went through Cyrodiil fast enough, it's more than feasable for them to reach the Imperial city before reinforcements arrive. In fact, if I'm not mistaken, reinforcements from Skyrim and Hammerfell only arrived for the retaking of the Imperial City after the Dominion had captured it.