Because they had a war with eachother. Since the Castro Revolution in 1956, Cuba has remained America's most enduring nemesis, an intractable symbol of communism, a flashpoint for Cold War conflict and an unknown problem for the future. It can be argued that America has become a political victim of its own Propaganda, while Cuba has been a victim of geography and US policy.
Yes, non-US citizens can travel to Cuba. The requirements for them to do so typically include a valid passport, a tourist visa, and compliance with Cuban entry regulations. It is important for non-US citizens to check with their own country's government for specific travel guidelines and restrictions to Cuba.
Citizens of the US are not allowed to travel to Cuba under normal circumstances.
The State Department prhobits US citizens from traveling to Cuba without receiving authorisation to do so from the State Department first.
Cuba
President Eisenhower cut off all trade with Cuba, all travel between the US and Cuba, and made it illegal for American citizens to visit Cuba.
Yes, US citizens can travel to Australia.
Yes, US citizens can travel to Japan.
Yes, US citizens can travel to Singapore.
Yes, US citizens can travel to Uzbekistan.
Yes, US citizens can travel to the Philippines.
Yes, US citizens can travel to New Zealand.
Because despite the abject poverty into which Cuba was plunged by the US Trade Embargo and the ludicrous travel restrictions within the US for people who have been to Cuba, Fidel still spends more per capita on his citizens than the good old US of A.