My best guess is that a Quick Release axle does not extend out past the drop-out and there fore the main rack mounting straps have nothing to rest/mount on. I suppose you could get a longer OR axle and custom cut it to a length that would allow the proper mounting point for the rack. The axle needs to extend out past the drop-out the thickness of the rack material on each side. You may need to get a longer QR Skewer as well.
probably because of the motor support arms that connect to the rear axle. i'm presuming that quick-release axles are too short.
more than likely, quick-release hubs CAN be used, if you modify your support arms to connect to the frame, not at the rear axle.
The problem is not that the axle itself is not strong enough--the problem is that when mounted on a QR axle, the engine's mount is not sitting on the axle, but on the QR skewer, which is much thinner. The axle of a QR setup usually does not protrude beyond the frame's dropouts.
I put a GEBE on a QR-axle bike (for a while) and I can tell you one problem that you run into: the notch cut into the bottom of the motor mount is sized to straddle a regular axle, which is about 9mm dia. The engine sits too low when straddling a QR skewer and the belt tensioner won't work right. If you use bushings to raise the engine properly (as I did) then the engine's weight is sitting not on the hollow axle, but on the QR skewer itself, which is much thinner--about 4mm--and (I would bet) could be eventually broken by this use. I had mine together and it worked, but overall I'd agree it's just not a great idea.