alt Hacker NewsThere a lot of discussion here https://retrocomputing.stackexchange.com/questions/7412/why-... but nothing seems conclusive.. I would wager the last answer, "IBM was using 400Hz", to be most directly causal reason. The motor-generator configuration might provide galvanic isolation and some immunity to spikes and transients as well?
Smaller transformers and capacitors in all the linear power supplies. 400Hz is still common in aircraft. Distribution losses are higher, but you're going across the room, not across the country.