First off, China, Cuba and Korea are not and never have been communist. The only countries to have ever been communist were the USSR 1917-~1960 and Albania 1945-~1985.
Explaining why in full would take far too long so I'm going to keep it brief - all the other supposedly communist countries (as well as the USSR and Albania themselves after being taken over by revisionists) were explicitly founded as states in which all classes would hold power together in an alliance for the common good, led by the proletariat. However, the true Marxist-Leninist formulation is that of the dictatorship of the proletariat i.e. ALL political power is wielded by the working class, none whatsoever by any other classes, which over time will be either abolished by force or absorbed into the proletariat.
Now for the answer to your real question. The reason is that no society which ever has been or claimed to be or aspired to be communist, socialist or even progressive has ever been left in peace by the bourgeoisie to the extent that it's been able to develop how it wanted to. I'll give one brief example.
When the USSR was first founded it has fully democratic elections, opposition parties, all kinds of civil liberties and even gay marriage. Most of that was abolished in 1920 - why? Because every important country in the world invaded the USSR, including but not limited to Britain, France, Germany and Japan. This invasion was accompanied by aid to terrorist groups within the USSR. On top of this, large numbers of the Russian Bourgeoisie actively infiltrated the Communist party with the aim of destroying socialism.
People talk a lot about Stalin being paranoid, but don't usually ask why. Here's an example - Boris Bazhanov was the highest level defector in the USSR's existence. He was a member of the Politburo and Stalin's personal secretary before fleeing to the USA. In the USA he wrote a book and here's what he had to say:
Starting in 1920, the open struggle against the Bolshevik plague ended. To fight against it from outside had become impossible. It had to be mined from within. A Trojan Horse had to be infiltrated into the communist fortress .... All the threads of the dictatorship converged in the single knot of the Politburo. The coup d'état would have to come from there. ... To spit at them in their face would have only given me 10 bullets. I took another path. To save the élite of my city, I covered myself with the mask of communist ideology.
He was far from the only one.
So basically, there was repression because there were groups and individuals who ought to have been repressed. Repression isn't automatically bad in some Manichean way - it depends who is doing it to whom and why. If it's the working class repressing those who would send them back to slave in the fields to support a few parasites then repression is a very good thing.