Tell me you're a dumbass without telling me you're a dumbass
I cannot see what strategic reasons these might be, or why they would justify the massive costs of invading a NATO and EU member state.
I am, of course, aware that the US president is a megalomaniac. But I think you are thinking too much from a political perspective: the US has been a plutocracy for decades. Therefore, the economy pulls the strings, and I cannot see what the massive losses that an invasion of Greenland would entail would justify - there really is no economic gain and that is all the US is about.
You want a short answer: it would cost the US economy billions to attack Europe, and that is why they will not do it.