Because poor people don't care about them if their lives become worse.
That's the sad truth.
I mean, they do care about them, but their lives already suck a ton so distant doom is not really on their radar when faced with constant, present doom.
Simple example: EVs are arguably less polluting. So there might be a need for a policy to ban ICEs, which are much, much less expensive monetarily. That would mean poor people wouldn't be able to buy and use cars, at all. So poor people will obviously vote against that. It both makes sense and it's sad at the same time.
Oh, to clarify: rich people also pollute a ton and don't care about the environment, but they have the luxury of not caring if environmentally sound but more expensive policies are adopted (they can go "whatever" and do the right thing with no major personal loss for them).
On top of that, it is the rich people's responsibility to find environmentally sound policies which are acceptable to everyone, since they're the ones with most of the power.
Edit: Oh, you don't have to believe me. Go talk to 100 low income people (construction workers, cooks, waiter, nannies, agricultural workers, etc) about the environment, it will be quite eye opening for ivory tower folks.