From: IDGA

… U.S.’s often overlooked northern border has some very serious challenges of its own and there are real and very serious criminal and terrorist threats along its long frontier.  The U.S. Canada border has been called the worlds friendliest and for a long time many Americans viewed crossing the border into Canada on par with crossing state lines.  It has commonly been seen as very easy to cross, low hassle, and having minimal security presence even at checkpoints.

For those very same reasons this same border has been becoming increasingly attractive to criminals and terrorists that see great opportunity for smuggling and low chance of being apprehended.  The U.S. Canada border is massive in terms of mileage and greater in size than the southwest Mexican border that is very large in its own right.