There are a couple of reasons; Karl Marx was an atheist and expressed this in his work. This obviously would influence "Marxist" thinkers.
More practically, the Church in communist states would have been seen as "competition" to the government.
Under some dictatorships "atheism" is what is expected by the Communist regime eg: Russia before glasnost
Communist Party
Communist Party of Vietnam was created on 1930-02-03.
How did communist schools benefit the state and the communist party
How did communist schools benefit the state and the communist party
New Communist Party of the Netherlands was created in 1992.
Lithuanian Communist Party declared separation from the Soviet Union's Communist Party.
How did communist schools benefit the state and the communist party
the Communist Party of the Soviet Union. But in fact it stood for state capitalism not Communism
The Communist Party of Spain (PCE) was founded on November 14, 1921. It played a significant role in Spanish politics during the 20th century, particularly during the Spanish Civil War and the subsequent Francoist dictatorship.
The Church feared the state-imposed Atheism of the Communist doctrine.
Communist Party of Iran was created in 1984.