Because it's not the goal that's in dispute, it's the difference of ideas about how to achieve it.

I can't prove it, but I'm convinced it's due to lobbyists.

In the past it was the right who owned this issue. Conservationist was the term. Now it's environmentist and seen as a left value.

Pretending that your position is the center, and everyone who disagrees with you is an extremist doesnt work very well.

The right doesnt oppose clean air, it opposes giving Al Gore's carbon credit company a blank check.

The right (in the US) could just as easily ask why has boarder security become a left right issue.