The only potential reason I can see is that it could be strategicly useful as climate change makes the weather there more mild. Given that this is Trump, however, and Denmark (and thus Greenland) is currently an ally, it would make more sense that he read it Twitter or something and he repeated it.

Trump said he’d take control of Panama and Greenland potentially including a military invasion, and said he’d buy Canada.

Thats the short, out-of-the-loop, reason you’re hearing about it.