I can only speculate, but perhaps it has to do with the regenerative braking the boards provide. On an e-bike you still have friction brakes, but on one of these boards you need to be able to brake quickly using the electric motors.

More powerful motors, while perhaps not strictly necessary, would also provide quicker acceleration. For a product that's designed to be fun, this might matter more, vs e-bikes which are mostly designed to be primarily utilitarian.