Canada has roughly the same HDI ranking as the US, whereas Mexico is somewhat lower. So from the "looking for a better life" perspective, Canadians don't have an incentive to move to the US (other way around actually, from HDI).
Just a guess though.
Well yes, but so is Canada, which has a higher HDI than the US.
Parent was asking why Mexico is excluded from the list while Canada is not.
By "don't have incentive" I'm just referring to an on-paper incentive from an HDI ranking.