It was not "trading electricity cost for BTC". It was scientific research, trying whether the system works.
Probably on computers running anyway. They just generated BTC while doing what they _wanted_ to do anyway, building and using and testing the system, out of curiosity and believing in the idea, not because they thought "that BTC gonna be worth my power cost". They just generated BTC in the process of figuring it all out, and afterwards, they had something other could not copy without at least a little effort.