Because that's the point of government: To govern.

Edit: We don't always let people pay "market price" for whatever they want. There's plenty of precedent for rationing: IE, In CA, it doesn't matter how rich you are, you can't water your lawn in a drought. During the 1970s oil embargo, it didn't matter how rich you were, you still had to ration gasoline.

In this case, cryptocurrency is both increasing the cost of electricity for other important needs, like home heating / cooling, and increasing pollution. It's perfectly within the definition of what government does to set reasonable limits for the cryptocurrency industry.