TL;DR: Koch Brothers, Citizen's United, Corporate takeover of the GOP.
Excerpt from The New York Times: The campaign ad appeared during the presidential contest of 2008. Rapid-fire images of belching smokestacks and melting ice sheets were followed by a soothing narrator who praised a candidate who had stood up to President George W. Bush and “sounded the alarm on global warming.”
It was not made for a Democrat, but for Senator John McCain, who had just secured the Republican nomination.
It is difficult to reconcile the Republican Party of 2008 with the party of 2017, whose leader, President Trump, has called global warming a hoax, reversed environmental policies that Mr. McCain advocated on his run for the White House, and this past week announced that he would take the nation out of the Paris climate accord, which was to bind the globe in an effort to halt the planet’s warming.