Really?
Back then they still weren't sure who'd get the nomination until the convention. Goldwater may have had the delegates, but people weren't psychologically prepared for the fact that everything could be decided in the primaries, so they fought on into the convention.
Now we know that Trump will be the nominee, so anti-Trump forces are accepting that -- or if they fight on, they know they can't win. People have time to decompress, so people aren't as keyed up as they were back then (I guess -- I wasn't around then).
The other thing is that Rockefeller really represented the elite. I guess you could vaguely compare the Bushes today to him. But they aren't carrying on the fight any more. Some political writer or consultant who's running #neverTrump doesn't compare to Rockefeller in power and influence.
Good point.
re: "they fought on into the convention"
Yes that's where it happened. It used to be great entertainment even before TV.