Because the US wants to economically isolate Cuba to prod the single party authoritarian regime into liberalizing. It's fine if you think that's a bad thing. My only point is that it's not a blockade, it's an embargo. Countries have the option to trade with Cuba and live with the additional tariffs on their exports to the US. Under an actual blockade, that option doesn't exist. The Royal Navy didn't let ships into Germany during WW1 and slap their flag countries with tariffs. No, they boarded and seized the vessels because this was an actual blockade.