Why convert to pesos?

I think that the film should have offered a better explanation as to why the US dollars needed to be converted into pesos. It's only been in the past couple of years (2010) that the Mexican government has decided to no longer honor US currency.
Before I checked this out, I couldn't understand why the bank robbers went to all of the trouble to travel to Argentina, but now it makes sense.

"Free to those that can afford it, very expensive to those that can't."