Historical, physical, engineering reasons.

Much of the world's mains-voltage electronics run at 240V (historical) and have PFC circuits (which are essentially just boost converters) that run at ~400V DC link voltages. 650V gives you enough headroom to tolerate overshoots and still have an 80% safety margin with a single level topology.

This voltage also coincidentally is a convenient crossover point where silicon MOSFETs start to become inefficient and GaN FETs have recently become feasible and mass-produced.