Your first sentence and the premise of your question is literally completely contradictory.
Fogive me, I’ll rephrase.
There have been multiple leaders of Socialist governments with communist political ideology. China, Russia, Cuba, Vietnam, etc are just a few examples of countries that embraced socialism in the 20th century with the goal of becoming communist. Now in the 21st century these same countries are straying from socalism and communist ideology and becoming increasingly capitalist.
Why is it that communism never materialized in any of the socialist countries of the 20th century?