It sucks, but I'm not surprised by this. It happens all the time with DVD series releases. Hell, the reason Wonder Years hasn't hit DVD at all is because all of the licensed music in the show.
And I'm not surprised by bad sales at all. I've got seasons 1-4 but I don't think I've even opened any after season 1. Why is that? Because I can probably turn on the TV right now and find an episode of Fresh Prince on somewhere. The show was sold off to so many networks and cable channels that it gave people little incentive to need to get the show for themselves.