Since when? We are still operating under Pegula's EEE guidance. He was told to get under and stay under the cap, after the purge of players who didn't want to be here he started using the cap underage to sign his own players with very few free agents. We all assume once we are a playoff team he will spend close to the cap. Right now he is still working the plan that he announced when he took over and that plan had restrictions.
I think you are saying KA isn't going to absorb some of his salary just to acquire a 4th round pick, which I agree would be his position - because he is under restrictions. VO is an UFA after this season so he could eat some of that salary, but he apparently won't or can't.
I think there are several reasons the trade didn't happen:
KA could not get what he wanted, then
Quinn got hurt so he decided to keep him, and
He is operating under restrictions such as retaining salary.