Secondly, they need a big bad boogieman to blame their failings on. Most western countries will massively miss their targets for cutting emissions over the next decade, 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!"
Also (1) Bitcoin usage is still far from mainstream and (2) PoW is too abstract a concept.
Regarding (1) it's easier to blame something that only a fraction of the population is using rather than something the majority of people, like driving fossil fuel cars, eating meat, short distance air travel etc. All things that get plenty critized of course, but also all things that politicians rarely dare touch for fear of public backlash.
Regarding (2) a lot of people simply don't get PoW. It's easy to attack something that is not properly understood and thus perceived as useless.
That being said, I don't think there will be any
meaningful legislative interference. Just how going after Bitcoin's PoW is symbolic rather than effective, any regulation "attacking" PoW is likely to be little more than posturing as well. And even if some governments would ban mining outright, the way China did, some other country will always take their place, assuming they'd even be able to effectively crack down on it to begin with.