There are lots of reason why the Cadets have no respect for de Guiche. First, he is a married man who is conniving to have an affair with another woman (Roxane). Second, he is the Cardinal's nephew, which does give him some sort of power and influence. Unfortunately, he chooses to wield that power for selfish reasons (to set up a clandestine rendez-vous with Roxane in a monastary, to manipulate which troops are sent to which battlefront). He is a coward who claims he's at war yet fights no battle. These are all despicable characteristics to the battle-hardened, honorable Cadets. The worst incident directed pointedly at these men is deGuiche's traitorous act on the battlefield. They did not behave toward him with the respect he would have liked, so he gave the signal for the enemy to attack them. Honor is above all, and de Guiche displays no honor at all.
I think the answer is because de Guiche did not portray honor at first.