The EPA was the idea of the progressives who seemed to have some momentum toward eventually achieving it. The conservatives (and their polluting corporate benefactors) were threatened by it. So, Nixon co-opted it while conservatives were still in power with the purpose of making sure it was ineffective. They achieved that.

You'll notice that when conservatives have power, they castrate the EPA and make sure it does not affect any of their benefactors. The Dem neo-liberals usually bend a little toward the progressives and will re-instate some of the EPA's power when the Dems are in charge, but it's never enough to undo whatever damage the last GOP administration did. This is because neo-liberals are conservatives. (They aren't as conservative as Republicans, but they are conservative by every international standard.)

Do not give credit to Nixon for the EPA. Like everything else ever done by a conservative, it was an act of self-serving deception. Nothing good in all of human history has ever come from conservatism. Nothing at all.