How long was NAFTA in place before China "entered the picture"?
China started to really enter the picture in the late 90's. After Nafta was pased in what '94? Do you think most companies were going to start building plants in Mexico the next day? No. Nafta was something new. Most companies did not know how to proceed to another country and were cautious.
Most were watching to see what other companies were going to do...and how they did it. Building a new plant in another country does not happen overnight. Or companies were waiting to see if Nafta was going to stand and not be repealed or amended and then their plant in Mexico would bhave been a bad business decision. Just as many companies have watched how other companies that have moved to China have done it and took their time moving over there.