It's simple: money and the local market. VW has been in Mexico a long time; VW is a global company that has plants in various South American countries, several countries in Europe, and of course, China (where VW has been since at least 1983, long before Toyota, GM, Ford, and Honda...)
Since the USA plant closed in the early 80's, all production for "volumne models," for the US market have come from Mexico in addition to producing models for the local market like the old air cooled Beetle until recently. But costs are high even in Mexico, so there is a constant rumour that VW will bring over a vehicle from China to sell in the US. It's a matter of numbers: eventually the Rabbit, GTI, and the Jetta will come from Mexico as soon as they get the issues of material quality in line with European standards. With the Euro trading at close to 1.40 US dollars to ONE EURO, the costs of importing the Rabbit and other models make it too high to make a profit. With most suppliers for VW already in Mexico or other South American countries, it makes sense to build the car in Mexico, where labour cost is a lot lower than in Europe... only China is less expensive and we may get a vehicle from there soon.
Volkswagen dealership service departments from 1980 till December, 2005.