It seems throughout European history that many of the trading or naval powers of the Baltic and/or Mediterranean had republics. Such as Venice, the Hanseatic League, Genoa, and so on. Are there any theories as to why this is or even if there is a correlation in the first place?
Sorry to get to this so late, but you might be interested in this answer I wrote from about a month ago and this answer on Italian urban republics.
The gist of the answer is that Republican governments favor trade and economic development, which is why you see these governments emerge in urbanized and densely populated areas like north Italy, the Netherlands, and the North Sea.