it would seem as though trump has different expectations from mexico and canada, and that this is neither about immigrants nor about drugs in either case. mexico has done what trump wanted and canada hasn't.
or maybe trump just hates trudeau and doesn't hate sheinbaum. that might be closer to the truth.
what this is about is offshoring taxation. they've been clear. canada is more wealthy, per capita, than mexico. canada is consequently a better country to tax than mexico. it might be difficult to actually do the math on that and have it make sense, but trump doesn't seem to be doing much math, or seem to be very good at math. this is likely something approximating the actual thought process.
tariffs on mexico would really drive up the cost of food in the united states. a lot. that would not be true about tariffs on canada.
trump is a bully looking for somebody's lunch to steal, and we're the easier target, with less consequences.