I am just guessing, but I'll bet this is done on purpose to avoid confusion about what happens when the subpanel is hosting a VI vs when it is not hosting a VI. Events on the subpanel tend to get routed to the VI it is hosting, so for consistency, events going to the middle of the subpanel tend to be dropped if there's nothing to receive them.
In practice, it probably doesn't save you much --- I can't think of a way to make arbitrary drag-n-drop work without having two procedures, one in the host VI that handles drag-n-drop when there is no subVI hosted and another in each of the subVIs to tell the host to change. Ug.
For the one in the host, I would suggest you should put a gray picture control over the top of the subpanel, use that as your drop target, and when it gets hit, hide the picture control. If you don't need to actually click on the subpanel, you could just make the picture control transparent and then leave it in play. But if you need to interact with it then any further drag-n-drop would need to be detected in the subVI.
You *might* be able to make it work using "mouse enter" on the subpanel and using that to trigger unhiding the picture control. I don't know how the timing on that would work out -- unhiding a control might not cause the events chain to reevaluate its drop target. Or it might. You'd have to test.