Answer:
Simple, because they are communist
Step-by-step explanation:
Back in the Cold War, Cuba suffered a revolution with Fidel Castro, with help from Che Guevara, took over Cuba. The US, being the imperialist hypocrites they are, attempted to invade in the Bay of Pigs invasion, which failed, and the Cuban Missile Crisis happened (USSR places nukes in Cuba as a response to America putting nukes in Turkey and Italy.). Anyways, spoiler, the US never ends up invading Cuba, and Castro remained in power until his death in 2016. They still blockade them simply because they are still communist to this day, and that in my opinion is what causes suffering in the country. Not communism, but the sanctions and blockade on Cuba.