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.
One thing that really left a strong impact on me was the first time I heard an explanation of the Mass Line. So you're trying to build up yourself a nice little Maoist guerrilla insurgency. What is the first step to getting people on your side? Figure out what they want! You go to the people community by community, village by village, and you talk to them about their hardships and struggles. You gather them together so they can work though these problems and come up with solutions. With an emphasis on dialectical materialism, the solution to all of these problems will naturally circulate around extinguishing the malevolent influences of Capitalism. But you start small, and you work in that direction as the low hanging fruits are eliminated.
You aid the people in building organizations around alleviating these hardships and struggles, engender their trust, build solidarity, and expand to more villages, more communities. They key is, you're not trying to simply serve the role of a charity. You're trying to feed, shelter, and medicate the people - yes - but more importantly you are trying to cultivate a culture where communities are empowered to do these things themselves, to analyze problems to their root causes, act towards resolving them, and go on to help others. It is like a pyramid scheme except instead of selling knives to people who sell knives, you're empowering communities to empower other communities.
Elections are virtually nonexistent in this process - or at least, unemphasized and peripheral. Volunteers step up when they identify with the goals of the movement. People take on leadership roles because their contributions to the cause are recognized and respected. And yet, it seems to me like a much more democratic process for all the people involved than the pinball game we play with liberal democracy.