Korea and Vietnam are very close to China / USSR, and at the limit of US force projection range on the western pacific so if the US wanted to preserve its sphere of influence there they had to fight for it.
Cuba is well within the US sphere of influence even as a communist state because no other power can really challenge the US there - with the single exception of nuclear weapons hence why the US was willing to go to the wall on that issue since that’s the one thing the USSR could have done to make the Caribbean no longer a US province.
They will sanction and isolate Cuba because they’re ruled by psychopaths and the existence of Cuba challenges their ideological foundations, but they aren’t actually threatened by Cuba and their sphere of influence isn’t actually threatened by Cuba so it’s not worth the cost of invading.