Good question- I would guess that it's due to Mexican government mismanagment. Factories need infrastructure to support them and Mexico hasn't been able to get

Good question- I would guess that it's due to Mexican government mismanagment. Factories need infrastructure to support them and Mexico hasn't been able to get that into place due primarily to corruption and inefficiency. China has a culture of relatively efficient bureaucracy. This means that if a part of the country needs more water, electricity, rails, etc. to support factories, they get it. I bet now that Mexico's institutions are becoming more modern and since they have less transportation expenses to the US this will change very quickly and Mexico will be a manufacturing hotspot.