I thought it was common knowledge already by that point. He wants Canada's resources and strategically advantageous geographical location.
The US government is broke. The spending and debt are out of control and their political system has become too dysfunctional to fix it intelligently. He hopes that annexing Canada and exploiting its resources unhindered by regulation will magically solve the problem.
Canadians, do not fall for it. Trump wants you to work in his mines.