Maybe the recent cases of smuggling guns from U.S is due to the original sources of private and sanctioned sales to Mexico have jacked up the prices due to the struggle. It's all speculation, since public is not privy to this information.
U.S. Arms and Equipment Sales Listed By Country | Just the Facts
And you know, there is no better time to raise your prices on weapons when there is a conflict.
Also, I don't doubt that 90% of weapons are coming from U.S, but I honestly doubt it's from smuggling efforts that go directly to the drug cartel. I think the smuggling is a result of isolated gangs or street merchants trying to undercut the standard going rate in Mexico. I think with the reputation of how easily Mexican officials are bribed, it could be the direct source of weapons are from the Mexican government itself.
Or the C.I.A, they've been known to fund operations by selling drugs/weapons.
XI Wiki


