There are a lot of factors that have gone into this, including but not limited to:
1) "American sized" amounts of money being dumped into a "Mexico sized" economy.
2) (related to the above) The increased utility of any bribe to somebody living in Mexico. (we see this phenomenon in developing nations across the world^).
3) Comparative resources available to the police forces.
4) Pressure from the US to overextend the Mexican police force.
etc.
To boil this all down to "the police aren't doing their jobs" is a huge mistake.
EDIT: ^Apologies, it seems Mexico is now considered a "newly industrialized nation", as of 2011. I don't think this effects my point however.
1) "American sized" amounts of money being dumped into a "Mexico sized" economy.
2) (related to the above) The increased utility of any bribe to somebody living in Mexico. (we see this phenomenon in developing nations across the world^).
3) Comparative resources available to the police forces.
4) Pressure from the US to overextend the Mexican police force.
etc.
To boil this all down to "the police aren't doing their jobs" is a huge mistake.
EDIT: ^Apologies, it seems Mexico is now considered a "newly industrialized nation", as of 2011. I don't think this effects my point however.