Eh...overwhelming air strikes, without ground troops, leaves you with few survivors bombed back to the stone age or no gained subjects. Either way, what's the point, if you want to subjugate, and be the ruler of all? If you kill EVERYone, there's no one left to worship you. If you don't bomb them back to the stoneage, and don't use ground troops, you don't really have control, and Palpetine was a Control Freak.^The reasons are outlined in AOTC when Obi-Wan is touring the Clone factory.
That and the Battle Droids, with some exceptions (The Droideka), are shown to be incredibly stupid and fragile.
I haven't seen Clones in quite a while, but I don't recall this ever being explained. The tougher droids were never shown as being less reliable than the clones, who were cut down by the thousands.
I suppose it doesn't really matter anyway. Star Wars has always been pretty silly when it comes to war and combat. Why, for example, given their level of technology would any army on army battle ever be necessary. When you can bombard planets from orbit, it kind of makes you look dumb when you have thousands of individuals standing there in formation shooting each other in the face.
You can destroy a culture with air power, and even kill everyone and poison the well, but, you can't have actual control of what happens on the ground.
IMHO, of course.