Depends which "they" we're talking about. The old guard of the party didn't blink at 2016, but most voters and probably most younger party workers did want a change of direction (to varying degrees). Bernie was the most popular candidate in 2020, and Medicare for All was so universally supported that everyone but Biden (lol) claimed to support some version of it. It took a primary screwjob without modern precedent and covid throwing the race into chaos to derail all that.