The media was already turning on him. IIRC, he got better press as the upstart straight talker (reporters do love a maverick), but once he was clearly the frontrunner, there was a lot of news and analysis about whether he was electable, and whether radical MoveOn Democrats were taking the party too far left. Remember the whinging about liberal blogs in '06.
The clip was a pretty dynamic image in a medium that's starved for that sort of thing during political campaigns, and it became emblematic of the narrative that was building for at least a month up to that point.
If there had been anything similar with McCain in 2000, it's possible that would have caught on the same way, as much as reporters loved him. Remember the Bush campaign was pushing a "McCain has too much of a temper to be president" line for a while.