It's because Cuba was backed by the USSR's nukes, when it posed any sort of threat. After 1990, it was just sort of assumed that Castro's regime would fall apart on its own, since it no longer had Soviet support to rely upon (the US didn't account for the rest of the world still dealing with the island; they tried to stop that by telling US companies "no, you can't deal with them", but globalization bit that idea in the ass, didn't it?) But even so, without the Soviets, Cuba was simply no longer a threat, so it simply didn't matter who was in charge, and it still doesn't.
It's because Cuba was backed by the USSR's nukes, when it posed any sort of threat. After 1990, it was just sort of assumed that Castro's regime would fall apart on its own, since it no longer had Soviet support to rely upon (the US didn't account for the rest of the world still dealing with the island; they tried to stop that by telling US companies "no, you can't deal with them", but globalization bit that idea in the ass, didn't it?) But even so, without the Soviets, Cuba was simply no longer a threat, so it simply didn't matter who was in charge, and it still doesn't.