The thing about hitting the factories is you need to be able to hit most or all of the factories for a specific item. And Russia moved a lot of industry out of the Western area in world war 2. I don't know how much is still out that way but it's probably enough to limp along. And annihilating supplies in the supply dump forces a greater strain on the whole system simply because they need more trucks. By blowing up trucks and supply depots you can freeze entire divisions in place.