China is a much worse dictatorship than Cuba and yet we don’t have a travel ban with China.
China didn't seize the assets of US citizens, at least not yet. I'm sure if China decided to seize and nationalize all the investments that have been made in China in the last 30 years, we'd institute a travel ban there.
People (usually who are under the age of 45), forget the outrageous crimes Fidel Castro perpetrated against American citizens when he came to power. Hotels, restaurants, cash, bank safe-deposit boxes, homes and other businesses of US nationals living in Cuba were all seized in the coup. It is impossible (or should be) to normalize relations with Cuba until such a time that restitution is made to the families of those whose fortunes were confiscated, at least.