Have you ever worn the leg pads that they used? Aside from being far too heavy for a goalie to be constantly moving up and down, there was also zero padding on the interior of the leg to accommodate a butterfly move, and the pads didn't rotate forward the way they do now. They were on their feet because they had to be.
This is Patrick Roy performing a "butterfly" move circa 1984, using standard pads of that time. Notice that he's sitting directly on his legs, with his knees down into the body of the pads.
This is Patrick Roy performing an actual butterfly about a decade later. Notice how differently his legs are positioned, since the pads are designed to protect his knees in the butterfly stance. You can actually see the new "landing gear" padding slightly below and to the left of the second O in KOHO.
And then another decade later. See how the pads have now been molded specifically on the assumption that goalies will only use the butterfly. Thigh risers close the five-hole, the toes are molded to kick the puck out from a butterfly position, the pads are profiled to create a perfectly straight seal along the ice, and they're hinged to rotate forward and face the shooter on their own. And check out those shoulder pads compared to the ones that were designed for stand-up goaltending... the butterfly-specific leg pads enabled butterfly-specific shoulder pads, and next thing you know he's blocking the whole net from a down position!
It wasn't about goalies suddenly figuring out how to tend goal, it was about a technological change that opened up a different playbook. What was the change? The development of plastic composites which replaced the old leathery pads. The light new pads allowed goalies to move differently, which introduced the idea of dropping on every shot. That led to "landing gear", which made the butterfly possible. That led to the total re-envisioning of the size and shape of every piece of gear on their body with the butterfly in mind, which locked the next generation into playing a butterfly style exclusively.