This is from memory, of my involvement in formula E in the preparations for the first racing series. I was not directly involved with the FIA rule setting so may not be 100% accurate, but....
Formula E was from its inception, a racing series based in city centres with the audience in close proximity. The speed limit was in part to allow the track designers to specify the barriers and netting to ensure containment in the event of a crash, but also in part to manage the insurance costs for public liability in the event something went horribly wrong.
The risks are not just with the vehicle momentum, but also the fire risk from the batteries being damaged in the crash. Lithium batteries behave much like a lit firework if they are pierced.
In reality the cars have a peak speed of around 150 mph, but the actual speed driven is less than this, due to the need to manage the battery charge. That's where fan boost comes in, allowing the cars to use their true power for a short burst.