"Why are cubans in miami celebrating his death?"

Because they are a group of Cubans who don't like him. There are also Cubans mourning his death. Like all politicians some people liked his policies (because they benefited from them) and will be sad he's dead and others disliked his policies (because they did not benefit from them) and will be happy he's dead. Ultimately what he did or didn't do has no relevance to your question.