He wins it for making himself look an absolute rube by Kim Jong-Un.
And for selling out the US to Putin.
And for wrecking the Western alliance.
And for delivering a global trade war, or rather a trade war by America against the world (at least, including China, the EU, Mexico and Canada).
Why the Nobel Prize?
Because Trump's supporters have seen him achieve all of this and think it's worth a Nobel.
Go Trump! NMAGA! BGTANP!
Not making America great again! But getting Trump a Nobel Prize! - catchy slogan.