The problem of a democracy is that it’s largely unsatisfactory across the board. At most, you’ll get most of what you want, but that little bit you didn’t get can be the basis of radicalization. If we’d just been a bit more demand, less willing to compromise, we could’ve gotten it all! And when you get less that most of what you want, the population is left with even greater feelings of disappointment in their political leaders. Free speech will be used to exploit these vulnerabilities. Free speech can also alleviate the pressure of these vulnerabilities on democracy.
In contrast, a dictatorship basically demands people act like the decrees of the dictator are everything they want. Only uncritical speech survives in the open, while the critical speech is underground. It is much more demanding on a critical individual than democracy, because their public life must support the dictatorship, while their private life is filled of thoughts that oppose the public. And it’s not like they’re safe sharing their critical thoughts either, it’s always a potential avenue of self-destruction, both of the individual and of the dictatorship. As such, critical thoughts in a dictatorship must be policed strictly, invading personal privacy short of literally digging around in people’s heads, to ensure and reassure the ruling power that all is well.