In Marxism there's a concept called the "Dictatorship of the Proletariat". The idea was widely accepted among many early socialists. These included Vladimir Lenin, who took the concept of the "Dictatorship of the Proletariat" and melded it with his own plans to spearhead a workers revolution through the leadership of what he called a "Vanguard Party". In Russia, this was the Bolsheviks (translates to "the majority", as opposed to the "Mensheviks", the minority) who, during the Provisional Government after the 1917 Revolution shared duel power with the Provisional Government through the "Petrograd Soviet".

Once Lenin took control of the Russian government in 1918 he believed that the only way to effectively lead and maintain the revolution and protect it against the capitalists and foreign powers was through his vanguard party. All other parties where banned for their ideological differences with the Bolsheviks, and many joined the "White" (Monarchist), "Green" (other socialist), and "Black" (anarchist) Armies. Of course, Lenin died very soon after his revolution had begun, and his time in power and government was molded by the war-time model of "war communism".

Soon after, Stalin took power and took advantage of the single-party system to entrench himself as a dictator and command total ideological purity and suppress dissent to his totalitarianism. His and Lenin's ideas became the model for practically every other "communist country" of the 20th century, and are still widely held as the model for workers revolution by many today.

This is largely why democratic forms for socialism aren't the norm: historically, the only other examples of communist revolution are from countries which have been dictatorial, so they've become the accepted norm for enacting one. It's worth noting that Marxism is a very complex system of beliefs, and it's very easy for opportunistic men to distill Marx's sociological critique down to simplistic slogans and stir up violent mass movements to propel themselves to power on a "mandate of the people".

Virtually all which you refer to as "communist countries" that have existed through history endorse an interpretation of Marxism known as Marxism-Leninism, which was developed by Lenin, and which considers vanguardism central to the revolution — this implies that the most "class-conscious" members of society, which organize themselves into a revolutionary Vanguard Party, would lead the revolution and then manage the transition from a socialist state into a communist classless and stateless society. It could certainly be argued that this transforms the members of the Vanguard Party into an elite and an upper-class of their own, which naturally attempts to hold into power no matter the cost.

Before the Soviet Union rose as a great power in its own right in the decades prior to World War II, there existed a greater variety of Marxist organizations and parties across the western hemisphere which had produced their own theorists and developed the concept of socialism in different directions, such as towards democratic socialism or industrial syndicalism, but many of these organizations (such as the Industrial Workers of the World and the Socialist Party of America in the United States) were infiltrated and supressed by Western governments in the wake of the first World War and hundreds of their members and leaders were arrested and in some cases murdered, so when the Russian Revolution succeded, many socialists saw it as a sign that Lenin's theories were actually the most effective way to successfully overthrow the burgeouise state and establish a socialist one, so they adopted Marxism-Leninism and founded Marxist-Leninist parties which quickly swallowed their predecessors or pushed them into irrelevance.

With official support from the Soviet Union, these parties eventually became a mere instrument of Soviet soft power overseas. Many countries in which socialist revolutions were succesful during Cold War, the revolutionaries were directly supported by the Soviet Union, and as such, adopted a Marxist-Leninist position "by default".

It was not until after the fall of the Soviet Union that you could find more variety again in socialist movements and organizations worldwide, as without direct Soviet support, those that had endorsed Marxism-Leninism quickly faded into powerlessness and irrelevance themselves.