From everything I've read over time - it's not even the US that holds a grudge, it's been popular on both sides of politics to end the embargo.

But neither will since it means losing the massive voting bloc of Cuban-Americans in Florida, Florida itself often being a decider of US elections. They don't want the embargo to end and anyone who supports ending it losses their vote.

Their reasons seem to vary, from the worst being wealthy land owners who had their lands taken by Castro only wanting relations normalised if they get their land back to the more common escapees who don't want to normalise relations with the 'Castro government' ever, that Cuba needs to ditch the regime first.