Because there are (were?) enough former cubans who fled the country and want nothing more to see it burn, and their descendants, to determine the outcome of Florida's statewide elections, including their 30(!) electoral college votes and 2 senators.

Literally that's it.

I mean, it kinda just started working, after blocking the last trickle of energy to the island. Cuba introduced market liberalization and privatization reforms just couple of weeks ago, appears to be driven by the blockade.

Not saying the blockade isn't horrible.

Also consider what would happen if the blockade is lifted. A deeply, artificially depressed economy would spring into huge yearly growth that would last for a while. How's the narrative gonna look like? "Sanctions were the main cause of Cuba's suffering. Socialist regime achieves incredible economic growth and standatd of living drastically improves since the blockade was lifted." All that happening as socialism's approval rating in the US grows and self-described socialists are winning elected office. If I were a ruling class American capitalist, I'd never let that happen.