EPA spokesperson Jahan Wilcox apologized but didn't explain why the EPA withdrew its support. As he told Reuters in an email "It shouldn't be a surprise to anyone that [the EPA doesn't] plan to fund an awards ceremony on climate change." To start with, the administration's proposed budget for 2018 will see its funding cut by 31 percent, which will specifically affect its climate change and pollution initiatives. Even without the budget cut, though, it's hard to imagine the EPA supporting a climate change award in its current state.
Earlier this year, the agency pulled down its climate science pages to reflect the views of the White House. The president also signed an executive order rolling back climate policies approved by the previous administration. And let's not forget that the United States withdrew from the Paris Accord, an agreement between 142 countries to make an effort to reduce greenhouse gas emissions.
While the awards program for 2018 was canceled, the EPA's former co-sponsors, non-government organizations C2ES and the Climate Registry, intend to continue the tradition. They're now looking for a new co-sponsor willing to fund and host the program in the future.