Let me quote [1] "An estimated 90% of cocaine entering the United States transits Mexico."

The key word here is "transits". Every other country in Central America, and some in northern South America are part of that "transit" too. Mexican just happens, to its bad luck, to be next to the biggest consumer.

Nobody is arguing to have cocaine accesible by kids at the grocery store. The argument is to legalize it to regulate it and, as someone below commented, to bring the prices down so is no longer such a profitable business.