Probably you have the answer already in your post. When it's busy, APP might not fancy an overflight at FL80 getting in the path of departures and arrivals. But if it was quiet, there was no reason to ask you to change your routing/level. ATC usually tries to let you fly as you planned, and a good controller will appreaciate the fact it might be inconvenient to climb you in your type of a/c.