Marching in a dust storm is also a significant burden for the Empire's apes. Although they no longer need to worry about the pressure brought by the Federation's air supremacy, the short visibility distance makes it difficult for them to determine whether there is a Federation defensive line ahead in their direction of progress.
If they were to charge into a pocket formed by the Federation blockade, they might inflict some damage on the Federation before being annihilated, but it wouldn't align with their tactical/strategic intentions.
There are also a large number of mines, laid by the Federation's engineers using their previous advantage, on the main roads leading from the Mountain of Gods to the outside world. Unless they choose the most difficult terrain, they will have to clear the mines, which would slow down their march.
