Maybe because that would not stop mining, but would transfer mining operations to the black market.
I also have a theory that they prefer to have these mining operations in their territory, so they have a chance to exert influence over Bitcoin in the future.
Never heard of black market mining...How do you mine on the black market?

Those mining farms need huge amount of electricity, it's impossible to hide them, the government will know for sure.
And we talk about China where the government knows almost everything.
I they would choose to ban mining as they see a thread in importing energy (China is a massive energy importer) they will nuke those farms in a matter of days. They will not risk power outages hitting their factories because of bitcoin mining.
All the bitcoin mining rewards at 1 billion billion a month is peanuts compared to the trillion in goods produced.