The cultural development seems to have occurred extremely powerfully in Italy and almost nowhere else.
I understand that Italy had the only large cities in the world at the time, but what are the reasons aside from that?
Could you elaborate what you mean by "the only arge cities in the world" ? Because there is, obviously no way Italy litterally had the "only large cities in the world", or even in western Europe.
Don't you mean something closer related to the form of government of these cities ?
It didn't. There's a reason many people speak of the 'Italian Renaissance,' because there was another one: the 'Northern Renaissance,' centered mostly in the Low Countries. Think Old Master painting, Huygens, even at a stretch Leibniz.