Because he's a RAGING IDIOT, in case that wasn't clear.

So just to make things clear, we're all going to die. There's never a good time for Captain President Garbage Fire to try his hand at international diplomacy, but Trump visiting the United Nations after weeks of louder-than-usual thundering over how everyone's out to get him and everyone is stupid and etc., etc., etc., is bound to result in an outcome so embarrassing that it will take the United States decades to recover.

Far from restraining Mr. Trump’s belligerent tendencies, his senior aides are engaged in a quiet effort to avoid a direct encounter with Iran’s leader that he would be unprepared to handle or concessions that they fear could undermine their effort to keep pressure on North Korea.

So Biff's staff here is, apparently, focusing on two particularly urgent tasks. The first is they don't want him to publicly give North Korean dictatorship any more presents than he already has, and the second is that they don't want him meeting with Iranian president Hassan Rouhani at all because they think Rouhani would eat Donald's lunch before Donald so much as got his ass in a chair.

In the meantime, Trump is apparently deeply confused about why he's making the trip at all.

On Friday, however, Mr. Trump confused matters further by saying in a tweet, “I will Chair the United Nations Security Council meeting on Iran next week!”

No. No he isn't. That's not what's going on. Rather than admitting that Dear Leader was suffering from another bout of raging public dementia, however, Trump's team apparently "urged" other alarmed nations to "ignore" his tweet. Let's all pretend it never happened, everybody! Nothing to see here! We don't want you meeting alone with Donald or paying any attention to the proclamations he makes, but we assure you he is absolutely not unfit for office, the beneficiary of a widespread White House plan to hold the American government together with twine and packing tape until he is out of office or accidentally sticks his hand in a toaster, whichever comes first!

In conclusion, we're all going to die. And this is just the side show; heaven knows what will be happening in Washington while all this is going on.