* https://en.wikipedia.org/wiki/Render_unto_Caesar
Certainly the (Catholic) Church had a lot of temporal power, especially in the Papal States, but sovereignty was a bigger thing than in Imperial China or Muslim conquests or caliphs.
Huff also cites the legal framework (amongst other factors): the independence of the judiciary (even from the Church, even though clergy sometimes acted as judges) and a focus on a documented due process acted as checks on runaway power.
The Medieval Origins of the Legal Profession by Brundage is also an interesting read:
* https://www.press.uchicago.edu/ucp/books/book/chicago/M/bo56...
It even covers Roman law, and how it managed to be salvaged (to an extent) and be used as a base for European law, especially starting in the 1200s.
I haven't read Huff's book so here's my stab in the dark. Europe has a lot of mainland coastline and the Mediterranean, which favors the formation of many small nation states and trans-Mediterranean trade. Among a large number of diverse and competing states, political, military and social ideas are more likely to be employed somewhere, and if successful, will rise to prominence within the local trade cluster by the process of natural selection.
If you have a large number of independent nation states, many of which import products and ideas from the far East, one of them is likely to put together a new military innovation that gives them an edge in warfare. Since the area sees frequent competition, other nations will quickly come into contact with the new technology and be forced to adopt it, or risk swift decline. A period of rapid innovation in metallurgy, chemistry and navigation (to name only a few) would develop a political culture that values technological development over traditional ideals.
I guess this would increase the demand for scientists/intellectuals and engineers. The total fund allocated by the political elite towards academics either through patronage or universities would increase the number of scientists in Europe. Ease of communication through mail channels and access to wider array of materials, both developed due to the increased volume of trade, would allow more scientists, to do more complicated research and exchange ideas more easily than ever before.