Because Trump is the boss. It's the same way that most companies don't allow random department heads to speak directly to investors, but force everything to be filtered through the board meeting notes, and SEC filings.

What the EPA reports may be factual, but the presentation of facts, when they are spoken of, and how often, are all political practices.

Is it important for the EPA to tweet daily "hey guys, you know climate change is caused by humans?". No, we know it is and don't deny it is. What's controversial is: how do we reign in the problem, what are we willing to give up to do so, and how do we coordinate with our allies and competitors ?

The EPA's tweets may not be helpful in getting Chinese support, or encouraging a positive practical debate on how 3rd world nations can both improve their economies with limited technology and still be part of the climate change solution----or even if such a thing is necessary in the short term.