I am addressing directly why the Republicans want this, not why the current president wants this. Even if the current president changes, the Republicans will still pursue the annexation of Canada with financial interest.
Donald wants to bully anyone he can into submission; the requirements are designed to be obscene so that agreeing to them is akin to kissing the ring.
That and trying to get more willing sycophants installed in power whenever possible (a little gem hidden in the latest batch of requirements: Canada should stop protecting the French language status. If you know anything about Canada, you know that agreeing to this would be a political suicide for the party in power).
This is a thought terminating cliche. This is just the latest move in a long term strategic effort to undermine Canada's sovereignty. Zoom out, and that in it of itself is part of an even broader strategic effort to reorganize alliances which built the world order.
Everywhere there are criticisms of Trump, his administration, and their goals, there is always this commentary which not only fails to serve as an accurate representation of present situations, but more importantly, it serves to distract from the chronology of events which led us here, blinding the broader conversation from creating inferences of what will happen next which stifles the conversation from its most natural next reply: What are they actually doing, and what do we do if it does not benefit us?
Donald Trump wants to make the USA bigger. It will put him in the history books. He also wants a lot of chaos so he can be bribed to stop(he won’t).
There is no master plan, just greed.