After selling VW's for nearly 6 years, and being a strong proponent of anything TDI, especially a Tiguan, and after having asked time and time again about TDI availability in more models, VW's "Official" statement to the dealerships was something on the lines of "We can only produce so many TDI engines in the world. Frankly, we make more money on the ones we sell in Europe, so we are concentrating our majority of TDI engines where we can make the most return on our investment. With that being said, our Chattanooga Plant (Passats), will soon have a TDI engine facility".
Regarding the Polo TDI- I an tell you that the US isn't ready for it. I've had 30% of my Golf Test Drive customers get in a Golf TDI and tell me how small it was and opt for a Jetta, for less money. I can only imagine there are only a small number of people who would consider a Polo. The American mindset may have shifted to a somewhat smaller car, but the American "body-set" has obviously a need for a somewhat larger interior space.