Pruitt is a known critic of the EPA, and has gone so far as to file lawsuits against the agency in the past. He has made it publicly clear through his actions that he doesn't support protecting the environment when money is at stake, and he is on the record saying there is still a "debate" about climate change science.
His nomination doesn't come as much of a surprise, since the Trump administration has actually toyed with the concept of eliminating the EPA entirely – an agency the President didn't even know the name of while on the campaign trail. Trump said during a Fox News town hall in April he vowed to do away with the "Department of Environmental", and even gave the made-up agency its own acronym ("the DEP"…why the P?) before confidently assessing its impact on the US economy.
Pruitt is already geared up to eliminate the water pollution rule, as well as to do away with the Clean Power Plan, and Trump is set to sign the necessary directives.