This is my hypothesis based on looking at some of Communism's specific terminology and theory.
class warfare
. If you have a Manichean worldview where it's the workers vs the bourgeois, any one who disagrees with you is an enemy, an enemy of the people to be more precise. Or a reactionary. Or many other such terms which were used to sentence people to jail.
intermediate stage vs Communist stage
. If your system posits an intermediate, imperfect stage, then you could - but don't have to - reason that people should not hold your government accountable for any failures, since it has not - yet - reached the point at which its success or failure can be judged. "Let's reach the real stage, then we can hold elections".
dictatorship of the proletariat
. Words do matter and dictatorship has a particular, widely recognized, meaning. If your core tenets recognize a use for dictatorship, then why not use it, for the people's own good?
vanguard party
. These are basically enlightened people, who know better than anyone else. They can be a minority, in fact they are expected to be, but they will show the way.
You also have a situation where Communist-run countries tend to be vilified by the larger world. That could - arguably - have led to them not being able to succeed on their own terms. And it - certainly - has led many to denounce any opposition to their rule as base conspiracies caused by outside influence.
Still, those 4 items can be rephrased as "You're either with us or against us. Good things come to those who wait. We rule for your own good. We know what's good for you.".
Add to it that Marx's words are discussed and interpreted, ad nauseam, with a fervor and intentness usually reserved for the Koran, Bible or US Constitution.
Is it any wonder that this theoretical underpinning instills a tendency to ignore dissenting opinions?
Modern Communism, even if the question expressly requests that that not be taken into account, was brought into being under the particular circumstances of the 1917 revolution, in Russia against a system based on hereditary feudalism, under Lenin and then later on Stalin. Under extreme opposition by the WW1 Allies, who wanted to keep Russia in the war and landed expeditionary forces afterwards to punish them for not doing so.
Given different historical antecedents, maybe Marx-inspired governance would have developed differently, interpreted Marx in a more tolerant fashion and adopted different customs and attitudes. But that's not really knowable, not when Lenin and his successors have been so much part of the picture.
Still, none of this imposition of will over the wider electorate has to happen.
You could, in theory, have an elected Communist government that runs the economy on Marxist principles but submits to the will of the wider electorate every 4-5 years, without rigging the votes, without unduly limiting who gets to vote, etc... Fair elections, or at least about as fair as every one else runs their elections.
But this very rarely happens in practice with governments that expressly look to Marxism for their ideology. Nepal (from Obie's comment). Guyana, Moldova, Nicaragua, from a question I asked on SE.History.
That's about it: half a dozen times in 100 years. If it became more frequent and it was a custom, then yes, we might get Democratic Communism. For now, not so much.
p.s. Let's preempt some arguments.
- "But what about the other "democratic" governments? Are they all democratic?"
No, not necessarily. But if they claim to be democratic, they run multi party elections and occasionally lose them. Losing elections, and transitioning power, is a sign of a democratic government. One Communist governments have exceedingly rarely shown (but it has happened).
- "But what about the US and "outlawing" Communism?"
True, but most countries do not anything particular against Communism and despite that Communists have rarely won country-level elections. The Parti Communiste Francais, PCF, peaked at 21% and I'd wager that's been a pretty good outcome in rich democracies.
And, as much as the US goes out of its way to throttle Communism, this stricture is only aimed at one party and the other parties are allowed to compete just fine. Contrast that to a government run under Communism, where the general tendency has been to ban all parties not approved.
And, BTW, "people who vote for Sanders" are not "voting for Communism", not in any meaningful sense of the word. No matter what the Washington Post or they themselves think about it.