As far as I can tell, it's not so much a ban as that the current legislation just doesn't fit them. They don't fit into the existing category of "electrically assisted pedal cycle" and they don't meet the requirements of road vehicles.

Legislation needs to catch up. There is a European effort underway to create safety requirements and test methods for Personal Light Electric Vehicles that would seem like a good start [1]. Maybe we'll leave the EU just in time to not get the benefit :)

[1] https://www.din.de/en/getting-involved/standards-committees/...