All you really need is a government or society that isn't conducive to technological development, either because they persecute it or because they just don't do anything to protect and encourage it (e.g. no patent system or enforceable trade secrets).
Even today, what we see is that technological progress isn't evenly distributed. Most of it comes out of the USA at the moment, a bit from Europe and China. In the past there's usually been one or two places that were clearly ahead and driving things forward, and it moves around over time.
The other thing that inspires the idea of a permanent medieval society is archaeological narratives about ancient Egypt. If you believe their chronologies (which you may not), then Egyptian society was frozen in time for thousands of years with little or no change in any respect. Not linguistic, not religious, not technological. This is unthinkable today but is what academics would have us believe really happened not so long ago.
Not discovering sources of cheap energy and other raw inputs. If you look carefully at history, every rapid period of growth was preceded by a discovery or conquest of cheap energy and resources. You need excess to grow towards the next equilibrium.
None match that thesis
Age of exploration was powered by finding new sources of slaves and materials in the East (india, asia, also eastern europe to an extent)
The mongol conquest itself was capturing vast sources of wealth (easier to take from others than build yourself)
Same with Rome. Each new conquest brought more slaves and natural resources to the empire. It used this to fuel more expansion.
https://en.wikipedia.org/wiki/House_of_Wisdom#Destruction_by...
https://en.wikipedia.org/wiki/Library_of_Alexandria#Burning_...
https://en.wikipedia.org/wiki/Nalanda_mahavihara#Destruction...
https://en.wikipedia.org/wiki/Imperial_Library_of_Constantin...
https://en.wikipedia.org/wiki/List_of_destroyed_libraries#Hu...