When 'muh efficiency' Matters

 What's the difference between Hero's engine and Watt's? Efficiency. Watt's engine paid for itself, in a way Hero's couldn't. 

 Accounting. It matters, bitches. 

 Stuff like hot superconductors put options in the money. It makes stuff we could already do inexpensive enough to be worth doing.

 Without decent lathes, Hero's Greeks couldn't make decent pistons without an ungodly amount of work. Without coal, they would have to be wood-fired. Chopping down the tree and transporting the wood to the engine by hand would have consumed as much energy as the wood supplied. Mining coal is dramatically easier. If you have to use wood in the absence of rails and the absence of huge draft horses, you might as well do the work by hand in the first place. It's not that Hero couldn't have made a Watt engine, it's that it wasn't worth the hassle. Because muh efficiency.

 Another example: it's not that we can't mine from asteroids. It's that the metal would be ten or more times as expensive as terrestrial metal. It would be difficult to avoid consuming more metal during the trip than you can haul back. There's no reason to bother. 

 Further example: why did Newton and Leibniz both discover calculus? The need for calculus came up for everyone at the same time. It's not that the Babylonians couldn't have come up with differential calculus, it's just that it wasn't worth the hassle. They had nothing to use it on.