It's more than just the death penalty issue with Mexico. Prosecutors routinely agree to take the death penalty off the table with Mexico and with Canada. Mexico goes much further than the death penalty and even though we do have an extradition treaty with them they routinely refuse to arrest or do any follow up at all relating to finding our criminals if the criminal is a Mexican national. If the victims in the US were Mexican nationals they will help a little more, but if the victims are police officers or American citizens and the suspect is a Mexican national they almost never help. If the criminal is a US citizen that flees to Mexico, then for some reason the Mexican police never seem to have too many problems finding them and sending them back to us. It's only their own citizens they can't find.
How do I know all this you may ask? Spent 30 years as a California cop. Got shot in 1989 by a Mexican National who fled to Mexico and was never arrested or even looked for by Mexican authorities. We now have information that he has died there but because their records (that you trust) are so bad they can't even confirm for us that he's dead.