* Regulation. Plenty of countries are very specific about the exact mechanicals of brakes, in laws that are 100+ years old from when runaway cars killed people.

* Unsprung mass. Cars handle better if the wheels are very light and all heavy bits are in the body of the car.

* Large diameter motors are harder to design - there is no opportunity for a gearbox, so you lose one design parameter, and need the motor magnetics to be right around the rim of the wheel to make up for it.

* Difficulty of making the air gap within the motor very thin at such large diameters - you end up needing super large diameter roller bearings.

However, I think despite those downsides, the benefits outweigh them.