Ok, so not really my expertise but here goes:

First off, Rome did NOT have a democracy, it was a republic hence Roman republic. There have been arguments made that the plebian tribunes was somewhat democratic because it served to represent the "people" but in my studies, it was merely a moniker that showcased a certain political "faction." Leaders who would "fight" for the plebs were really using that as a way to advance their political power. This is a cynical way to see things, but it is far better than the idealist Romans=s democracy; being the pre-modern world, states were often run by the rich, for the rich.

Further, while Augustus is seen now as an emperor, he was never referred as such by his contemporaries; he was a "princeps" which means first citizen. He further pushed the notion that he had "restored" the republic after the civil wars. Obviously, in practice he was an autocrat, but the critical point is that he created the image of the republic continuing on; in that sense he was a de facto autocrat over a de facto senatorial government. This is important because it goes to your point about monarchy being stigmatized in Rome: Augustus went around it.

I also want to emphasize that there is evidence (from a few things I have read) that the Germanic tribes practiced some form of proto-democracy at a tribal level, but then in the later period would coalesce into monarchical states, no doubt with influence by the Romans. This is because concurrently after the 3rd century AD, we enter a period known as the Roman Dominate, and resulted in an overall increase in power of the emperors as they were no longer referred to as princeps. The reason is probably because after the 3rd century crisis, there was a need to increase state power in order to deal with threats.

Going to the crux of your question in why did it take so long to form republicanism in Europe: in fact there were republican states in the medieval world-- The republics of Venice, Genoa, Novgorod. In that way, republicanism DID exist in Europe at this time period. I know that Venice would last until the 1800s as an independent republic. By the 1600s, there was a growing trend towards increase monarchical power and the formation of nation-states; this is most readily seen in France and Spain (although I have heard that they did have a council of nobles who could overrule the king).

BTW, just because the Romans stigmatized monarchy, does not mean that everyone else who was conquered by them did as well; in the eastern Mediterranean, the emperor was often deified, and this process was exported to the western empire.