I'll throw in my .02c worth.

Bally was already having trouble getting enough current output and tried to overcome this with the Future Spa, KISS, and Space Invaders version power supply. Space Invaders did have the power supply in the bottom cabinet as well. Pretty much any of the 1st generation power supplies no matter what variant or where placed, toasted the connectors because of the current demands. Same with Stern who kept using the Bally-inspired power supply for every 6800 game they made (at least from my memory) in the bottom cabinet.

Bally needed more current and that meant bigger rectifiers, bigger transformer, more robust connectors, and more heat dissipation. I think the decision to move to the bottom cabinet was principally just more real estate to use.