That kinda tribalistic, us vs them false dichotomy is a big underlying issue for much of what's worst in modern politics. Is it so impossible to accept that people on the other "side" could ever be right in any way about anything? As for why people might want the "diet" version, what if in every other way they perfectly support the GOP party line, but do think that democrats are right, at least in part, on environmental issues?
It is a passionate, ideological subject we are talking about here, not whether VAT rate should be 6% or 8% or a compromise 7%.
If you accept the leftist narrative of climate change being as catastrophic and important as they make it seem, that stipulates certain trust to clearly left controlled institutions, and in turn certain quite strong policy preferences to be taken. Doing this stuff half assedly is just nonsensical, it is an ultimately unstable policy with no support. It is, barring minor details, all or nothing, other options are just hypocritical and irrational to both sides. If you believe it is literally an existential threat, you act accordingly. If you believe that it is no threat or a minor threat, you mostly chill, and the former group will protest your lax attitude all the time. If you chill slightly less in the name of compromise, the former group is still going to be very unhappy with you, so what's the bloody difference.
It is very similar in other debates regarding such highly ideological subjects - for example, abortion, one side believes it to be murder, other not at all, in fact they consider it a right that should be funded as part of public healthcare. What is the compromise to be had here?
That's why the debate there is also had over the big question mentioned, not whether first or second trimester abortions should be legal and publicly funded and the rest banned as a compromise.
Should they then go and vote against every other core value, to support the one issue?
How many people are willing to trust the media who are supporting the green policy on climate change, but at the same time turn around and say that these same sources are a bunch of leftist wackos in regard to all other political questions and they would rather listen to Fox News instead?
The idea that whatever one party believes or supports, the other party then has to take the diametrically opposed view, or at least a radically different one, without regard for the reality of the situation is a big part of why good faith compromise is now largely replaced by the ascendant party ramming through whatever they can only for the country to swing radically against that one or two elections down the line, and also why there's a great deal of infighting in both major parties. I mean, it's patently absurd to believe that one's opinions on birth control or religious freedom are inextricably linked to environmental concerns, or immigration issues. But because they're all hot topics, the parties have drawn up their battle lines, and a large majority of people are caught somewhere in the no man's land, forced to choose between two imperfect options.
The confusion is that some of the issues are not as ideological, or have more than just 2 main public stances on them with their own dramatically ideological lines of thinking supporting them. Compromises in such matters are more viable politically and logically.
But if they are so ideological, just dig down and consider the what is the ideological basis of that thinking, and then which party is more friendly to it. If its western cultural traditionalism, its gonna be GOP. If it is pursuit of economic equality of "protected groups" as defined by left leaning academia or similar socialist impulses, it is gonna be DNC, no doubt about it. If it is raw national self interest being promoted over foreigner's, especially poorer ones, you know the modern western left is allergic to that kind of argumentation (see: debates of refugees, NAFTA, UN), so GOP it is.
Where does climate change policy, which like it or not, is the hot topic that dominates environmental policy on both sides, more or less eclipsing other concerns (besides, many of the other concerns are naturally stuck in a similarly international idealism oriented corner, like anything to do with world scale biodiversity, or oceans), lie in that calculus?
Its very status as an important, severe and immediate concern is based upon trusting the line of left leaning institutions, including international ones, and the drivers of related actions are also naturally internationalist institutions, who choose to do it with a lot of leftie impulses in mind - China and third world countries getting major exemptions, increasingly common "climate justice" talk, distribution of carbon indulgences and so on.
That kind of stuff is just repulsive to right wingers of all kinds, from establishment GOP to libertarians and nationalists alike.
Nevermind the general theme of this policy implying lots of internationally controlled petty regulation of matters large and small and being against economic growth, and you know well GOP likes its economic growth, even the establishment kind, they are even willing to lock horns with the more ideological nationalistic wing in immigration debate over that.