DrunkenMage
Intoxicated Arch-Mage
The Legate in Morthal says something about it too. That they were having problems with recruitment from Cyrodiil so they had to recruit locally.
By the way, is it just me or have I just read this for the tenth time? Stormy skulls are too thick to pierce!
All Legate's mention that the Empire is spread thin these days. The Legions are stationed along the Aldmeri Dominion's border preparing for the resume of war, Civil War in Skyrim is a distraction caused by the Thalmor to divert men and resources and keep the Empire busy. It is why they provide indirect aid to the Stormcloaks and were also in Helgan trying to prevent Ulfric's death. Imperial victory will put the Empire in a stronger position, Stormcloak victory they want to avoid, but the way they word it;
The incident at Helgen is an example where an exception had to be made - obviously Ulfric's death would have dramatically increased the chance of an Imperial victory and thus harmed our overall position in Skyrim. (NOTE: The coincidental intervention of the dragon at Helgen is still under scrutiny. The obvious conclusion is that whoever is behind the dragons also has an interest in the continuation of the war, but we should not assume therefore that their goals align with our own.) A Stormcloak victory is also to be avoided, however, so even indirect aid to the Stormcloaks must be carefully managed.
They don't want either side to win the Civil War because it will harm their plans, though Stormcloaks aren't much of a threat to them, chances are they are far more concerned with the mass of Legions sitting on their border, than a group of Nords that can't reach them, chances are Ulfric will never lead an attack on the Dominion, he'll probably spend the rest of his life within Skyrim. Skyrim isolated and alone will take even longer to recover than with Imperial victory. Stormcloaks will be ready for a war, but in probably like fifty or even a hundred years, bit of an amusing concept for those who complained the Empire is taking too long to fight the Dominion, so their solution is to wait even longer.