I would imagine that people are calling these governments illegitimate because they lack a democratic mandate to rule and independent opposition to that rule.

I would imagine that people are calling these governments illegitimate because they lack a democratic mandate to rule and independent opposition to that rule. i.e. they weren't voted in; they ceased power by violence. You can't vote them out. No one has freedom of association to form an alternative political party that strongly opposes the status quo. Without those things one party governments can't say they are legitimate, only vanguard dictatorships.