Why has the US border with Mexico been scrutinized so much more than the border with Canada?

I have edited your question to show what I think you are
asking...

The major reason that the Mexican border has been
patrolled so much more than the Canadian border has to do with drugs and illegal immigration.
Both of these do occur on the Canadian border, but both are much more prevalent on the border
with Mexico.

The great majority of people who enter the US illegally
do so by coming across the Mexican border. That is because both Mexico and other Central American
countries are much poorer than the US. By contrast, Canada is a rich country with few people who
wish to enter the US illegally (though some Chinese enter by that route). In addition, the bulk
of "hard" drugs that enter the US come from the south. Canada is the source of some amount of
marijuana, but cocaine comes from the south (as does much
marijuana).

Because the Mexican border has traditionally been the
site of more illegal immigration and more drug smuggling, it has been scrutinized much more
heavily than the Canadian border.