Excess hydro capacity can be used for electrolysis to hydrogen, hydrogen can also be produced from flared gas. Which can then be used to produce low-carbon-emission steel with hydrogen as a reduction agent.
Bitcoin mining makes all that more expensive, less likely and risky, because setting up a few miners is quick and easy. Running a steel mill or electrolysis plant is expensive and longterm. Too risky if bitcoin miners might buy all the cheap intermittent power you planned with.
Your argument boils down to: why aren't people doing this uneconomically viable thing that I think is better than this other economically viable thing.
https://www.bloomberg.com/news/articles/2019-12-06/why-bitco...
Rather than venting greenhouse gases produced from the oil and gas mining process, they use it to mine Bitcoin. Sure this results in some CO2, but that's a lot better than venting straight methane.
The global market for adding bitcoin blocks is as competitive as it is possible to get. No barriers to entry, no one even has to know who or where you are. As such only miners who can secure the cheapest energy survive.
Excess hydro capacity in China during rainy season; capturing flared gas from shale oil fields in the US; geothermal in Iceland; any source of stranded or wasted energy.. bitcoin mining will soak it up.
Bitcoin mining using fossil fuels is simply not cost competitive.
This "#hashes => #GWh => #tonnes CO2", is broken logic full of unsubstantiated assumptions made by bitcoin haters.