A few ideas:
1. Technology forgotten or deemed unnecessary: cities in India had sewer systems millennia ago, but the technology was forgotten. Ancient Romans had steam power but it was used for entertainment and never put to more productive use. the Chinese invented the printing press in the Middle Ages, but their writing system limited its practicability. You can come up with lots of reasons why a technology might be discovered multiple times but never lasts or spreads very far.
2. Labor is cheap: <PERSON>'s answer is accurate in that small populations have less ability to achieve scientific progress, but the opposite can also be true. One proposed theory why the Industrial Revolution happened in Europe rather than China is that there was no real incentive for the Chinese government to invest in labor saving resources because labor was so plentiful! Combining with reason #1, China actually did develop many technologies like gunpowder and the magnetic compass, but they were underutilized. 1006
Why develop better weaponry when you can mass conscript? Why finance large trading fleets when you can produce everything you want at home?
3. Lack of resources: one reason why the Industrial Revolution began in Britain rather than anywhere else in Europe was because of the plentiful coal deposits in the country's north. The first coal powered engines were incredibly inefficient, but it didn't matter, because they were only used within and very close to the coal mines. It was only after experimenting and tinkering with these engines for decades that they could extend rail networks across the country. Germany and the United States became industrial powerhouses after Britain because they also had the resources to do so. Though you could argue that a country like France had better education and infrastructure, limited access to the proper resources meant they were always lagging behind the British and Germans when it came to industry.
Ultimately, there are a lot of potential reasons why technological innovation happens or doesn't happen, and historians don't agree on which factors are most important, so try mixing and matching!
Also, one quick note: Medieval Stasis is itself kind of a misnomer. Though there was a major societal collapse after the fall of the Roman Empire, there was innovation during the Middle Ages, but it was in less flashy things like better stirrups and plows. ;-) 1006