Two reasons. First, they need some angle to attack bitcoin. It has been attacked for being worthless magic internet money, which has been proven to be false. It has been attacked as being a scam or a Ponzi, which has been proven to be false. It has been attacked as being too volatile, too risky, dying hundreds of times, all of which have been proven to be false. The latest attack is that it is killing the planet. This is also false, but it is the angle they are using for the time being until education wins out and they pick a new angle.

Secondly, they need a big bad boogieman to blame their failings on. Most western countries will massively miss their targets for cutting emissions, because they have failed to invest in renewables, continue to invest in new oil and gas, and continue to allow private companies to pollute with abandon. Rather than turn around next election cycle and say "We failed because we accepted bribes from oil companies", much better if they can say "We would have succeeded if not for those pesky kids bitcoins!"

I am still hopeful that education will win out. It's not difficult to prove that bitcoin's energy consumption is completely insignificant. It's also starting to be recognized by various governments that bitcoin mining is unique placed to help facilitate a move to robust renewable network via what is known as demand response. See my posts here and here about this.

And even if we are stupid enough in the US to fall for this nonsense and ban mining outright, there is absolutely zero chance of a global ban. We already saw how quickly the hashrate recovered and starting hitting new ATHs again after the (ninth?) Chinese ban on bitcoin. And with bitcoin mining being one of the lead users of green and renewable electricity in the world, I'm not too concerned.