The US police/government/courts are less corrupt and have more resources to crack down on the more visible violence.

There is more violence in America than most people realize. It's considered mundane so you don't see it in the news as much.

The US is selling arms to the cartels in Mexico, so their violence is escalated there.

Mexican cartels aren't larger in the US than in Mexico because their supply sources are in Mexico, they lose momentum over distance and face domestic competition as they move across the US.