There are a number of reasons.
The Monroe doctrine. To paraphrase, James Monroe said that European interference in the political affairs of the western hemisphere would be treated as a potential threat to the United States. The monroe doctrine was the basis for the US blockading Cuba during the missile crisis. As political manuvers go it's a very dick swinging move and not something we'd tollerate from anyone else (we meddle all over the world). It's also a much bigger threat today than it was in 1823 (i think) when it was introduced if not because we are now the most powerful nation on earth, then because an attack on the US ropes all of NATO into the fight.
Political upheaval in the the US, Mexico, and Canada have been staved off by generally good economic conditions internally and with each other. You generally don't have revolutions from well fed people with stable jobs and you don't fight with your key trading parters.
With the US disallowing outside hostilities and the big players being very stable (overall if not on a micro scale) you really only get smallish fights between smallish players. Civil conflicts or things like DR and Hati bumping heads in the past.
Lastly, No one on Earth has the ability to attempt to take a fight to the US on a large scale. An attempted invasion of the North American mainland would end in crushing defeat. We have tactical superiority by land air and sea, and even if an enemy managed to make landfall in the US defeating the combined military aparatus of the NATO forces and the US military with a home field advantage, they'd be met with a populace of 300 million people with 400 million guns. Suburban police dapartments with armored vehicles, and tanks FFS.
Currently the biggest threats to the security of the west are the Cartel wars going on in Mexico, or the potential of a dramatic escalation of militarization at the Mexican border. I don't think either is a major threat at the moment, but things can change rapidly.