Why don't prices of USA labor and Mexican labor equalise?
The underlying reason for persistent wage differences is probably that Mexican workers are less productive than American workers.
It is sufficient that workers in some tradeable sectors are more productive to have persistent wage differences in all sectors. (a good explanation of this effect can be found here: https://en.wikipedia.org/wiki/Balassa%E2%80%93Samuelson_effect#The_effect_in_more_detail)
One would expect that unit labor cost in tradeable sectors converges, not necessarily wages.
Here is a graph that suggests this has already happened: https://fred.stlouisfed.org/series/CCRETT02MXA661N, for an explanation of real effective exchange rates see here: https://en.wikipedia.org/wiki/Effective_exchange_rate