If I were to guess, I would say it has a lot to do with velocity and a different approach from hitters. There were big strikeout guys like Dave Kingman and Rob Deer, but those types seemed to be a lot rarer than today. Most guys went for contact over power and that was valued by GMs of the day.
Looking back at the best pitcher of all time by WHIP, Addie Joss struck out only 3.5 batters per 9. It's easy today to think that strikeouts have always been the best way to judge pitchers, but that wasn't always the case. I'm certainly not making a case for Vuckovich, but it's so interesting how perceptions of pitchers have evolved.