Like allowing multiple political parties, full freedom of speech and assembly, abolishing the police, ownership of weapons, direct democracy etc.

The common justification is that they were in a dire situation where allowing too much freedom would allow counterrevolutionaries and foreign imperialists to sabotage and destroy them. I find this unconvincing, to what extent is security better than freedom? To what extent can the current leadership be trusted to "protect the revolution" than possible others better suited who couldnt take power?

Even then, why did the Soviet Union and other communist countries not democratize after WW2 when they arguably established sovereignty with their nuclear weapons?

Just as the capitalist ruling class preferred fascism to losing their power to communists, it seems the Marxist-Leninist rulers preferred capitalism to a more democratic form of socialism.

We see this happen now in Cuba, the last bastion of Marxism-Leninism, where the ruling class has been gradually introducing privatization and market reforms rather than allowing things like open elections, freedom of speech etc. Under capitalism, they can still rule.