It should be noted that democracy predated European feudalism, though it was not necessarily democracy as we know it. For starters, there was no universal suffrage, and in the case of the Roman Republic (where wealth and power tended to run in families), more democratic measures by modern standards were considered populist and thus anathema - though at the same time, the concept of tyrannicide was praised and Kings were abhorrent, which is why the early Emperors tended to be Just the First Citizen.
Representative democracy, likewise, was practically unknown in the Roman era: only Roman citizens physically present in the capital city-state had the right to vote in elections and governors were appointed by Rome to oversee the provinces. The concept comes from the late Middle Ages when the Norman kings of England formed the first parliaments of regional nobles to advise them and the Holy Roman Empire adopted a system of Elective Monarchy whereby the Emperor was selected by a council of Elector Princes (some of whom were themselves elected by their own vassals).
A common justification for a feudal interstellar government is the presence of relatively slow Faster-Than-Light Travel without a Subspace Ansible, so that in crisis situations a local government has to act autonomously. However, when travel is on the scale of weeks or even months, this disregards the fact that past governments have been able to function as federations or empires with similar limitations.
Other justifications may be the gradual ossification of society, capitalists becoming aristocrats over generations, a coup directly into a functionally feudal society, or a slow transition into feudalism by a combination of factors. Related might be a change in the meaning of titles similar to how Imperator went from a military honorific to a hereditary monarch while the Latin word for "first citizen" (Princeps) became Prince.
