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.