Why? There will still be criminals tradficking drugs to the US... They just won't be ceiminals in Mexico any more!
One big benefit of legalization of drugs was that the organized criminals wouldn't be able to make money off it, but most of their business is in the US anyway.
The crimes Mexican people most want to reduce are the violent crimes, the corruption, etc, all caused by people having to deal directly or indirectly with cartels if they want to produce/sell/consume drugs.
As long as the US keeps drugs illegal in their market, people will have to deal with the cartels to be able to sell there, and the US is the biggest market.