Honestly there isn't a legitimate reason.
Tldr "tradition" and insecurity, probably.
There was once a issue with Cuba a long time ago, but that got resolved and the US just decided to keep punishing Cuba as, in my opinion, a warning to other neighbors who would dare oppose it. "help out our opponents and get smothered"
The only reason it's continuing is because of weak administrations thinking that admitting people who ran the country in the past were wrong somehow makes them or the country look weak.
Honestly the only thing that seems weak to me is continuing to kick someone who's been down for decades but still manages to find areas to flourish and be better than the "superpower" that's kicking it.
People seem to think that the UN is a military force that is supposed to go in and secure democracy around the planet...
It's basically a chat room for all the countries to talk for everyone to hear. That's 99% of its job. And it does it quite well.
The problems occur when individual members decide "nah we don't give a shit about the UN right now" and usually it's the big ones that ignore it. Russia, US, UK, etc