

Well, there are a couple of differences there. First, Trump is an excellent media manipulator. Every moment that pundits thought would be a campaign-ending gaffe became free publicity. He got the equivalent of $2 billion in free media coverage from CNN alone.
Second, Trump didn’t actually say or do anything that would upset the donor class like progressives do. He was more vulgar and crass, and the RNC was certain he would cost them the election, but he wasn’t an existential threat to billionaires the way Bernie was.
However, if you want an example of how the RNC behaves when someone like that is running, look at the 2012 Republican Primary. Mitt Romney was the frontrunner, but the base was unenthusiastic about him and looking for someone different. Ron Paul polled in second place literally the entire campaign cycle, but the pundit class gave him no coverage. They wrote endlessly about Chirs Christie, Rick Santorum, and even Herman Caine, all of whom had brief moments as the frontrunner, but they completely ignored Ron Paul. His staunch libertarian beliefs threatened the defense industry and Wall Street, so the media and the party just pretended he didn’t exist. (For the record, Ron Paul was a wack-job and I’m glad he never became president, but the Bernie parallels are strong).
I’m not claiming Republicans are environmentalists, but if you want to know why they got so much worse on the environment, the answer is the Ratchet Effect. The thing you misinterpreted as, “both sides bad,” explains exactly how we got here. In Nixon’s era, environmental issues weren’t considered particularly partisan. Nixon, Ford, and Carter all had generally the same outlook on using the federal government to regulate corporations on the environment.
Then comes Regan with a lurch to the right. He tries to de-fang the EPA and hundreds of employees resign en mass. But he’s not all bad; he is instrumental in passing the Montreal Protocols, which effectively fixed the hole in the ozone layer, but he’s much worse than his predecessors. H.W. Bush was a little worse than that. He continued Regan’s deregulation campaign, and while he held several climate summits, he made no substantial moves on the climate.
With Clinton, we can see how the Democrats stopped the Party from moving back to the left on environmental issues. Clinton was, economically, very similar to Regan and Bush, and placed the corporate profits above the environment. He tried to make some progress with the Kyoto Protocols, but it was mostly ineffective, relying on cap-and-trade policies that did little to reduce emissions. Then it was the next Bush, who pulled us back out of Kyoto and was generally worse on all fronts for the environment. Next came Obama, who certainly has a mixed history on the environment. He put us in the Paris climate accords, but also went heavy on coal and fracking, plus approved the Keystone Pipeline. Finally we get Trump, who is a climate change denier and Captain Planet villain, which was interrupted by a brief interlude from Biden, who put us back in the Paris accords for a few years but also expanded American oil production.
Do you see how, over time, the Republicans move farther and farther to the right on the environment? Do you see how the Democrats fail to bring us back to the left when the retake power? That’s the Ratchet Effect. Democrats aren’t nice environmentalists that just want to fight the evil Republican polluters, they’re constantly shifting right with the Republicans. This is true for immigration, the economy, crime, and if guys like Gavin Newsom get their way, it will soon be LGBTQ rights as well. Your binary, black-and-white view on these issues just doesn’t reflect history or reality.