It was not that the Galactic Empire could not have fielded a vast, overwhelmingly powerful and incomparably competent armada against the Rebellion, it was that Darth Sidious did not dare to do so…..
I am more inclined to agree with Zen.
In politics, the enemy is behind the leader not in front. So it wouldn’t be in Darth Sidious interest politically to assemble a force large enough that it might splinter and become under command of another political force.
I would take it even further and say that all war is about economic considerations, and fought by people of little economic consideration.
With that said, my guess is that it was the same people funding the insurgency, and giving them the resources necessary to build its base on the ice planet, that were funding Darth Sidious.
In that way, a war on a distant planet and a war not to end all wars, Darth would not be destroying any more planets that had economic interest to the corporations, and if the insurgency gave a good enough showing (defeat was expected) more funds would be available to them as needed.
It is not that the corporations wanted Darth defeated (they loved his ability to literally control people), but they needed the markets to keep growing, and with a total defeat of the insurgency the market would tend to stagnate.
While it was necessary for a few economic leaders to give their life’s to the “cause”, most leaders within the economy, and their families, went untouched, and never really knew a war was at hand.