[PeterisP]

IIRC a key obstacle why steam engines were not used earlier despite the concept being known for at least a millenium was the requirement for quite advanced metallurgy - you can make a nifty proof of concept from copper or iron, but a useful steam engine needs to be (a) relatively high pressure and (b) large, so you can do it only if you can reliably and cheaply make large quantities of decent steel. If you can't make large quantities of steel, your steam engine doesn't work; if your steel-making process has unpredictable results, then your boiler blows up at a weak spot, and if that steel is expensive, you're better off having the same people work a literal treadmill instead of making a steam machine.