It's a country that is under the thumb of the USA. They are not allowed to develop in a way that's good for them. Right now, the fastest way in this oppressed country to get rich, is to smuggle drugs to the USA. And apparently guns are smuggled back the opposite way.
Obviously the guns that the cartels have are not legally held in most cases.
Mexico is truly the dirty side of the "shiny" coin of the USA, stuck in a VERY unhealthy symbiosis with the USA. Gun laws are certainly not at the root of the problem with violence there.