It's not as simple as you make it seem. A public poll won't work since car manufacturers wouldn't be happy about the negative press when they see their cars lose. And even if it's done more discretely, issues arise from user selection. PD probably isn't making car inclusion decisions on their own, I'm sure the manufacturers also have a lot of pull. In addition, even though I'm sure PD wants every relevant car in their game, time, money and man power only allow a small selection to be added every so often. And for every car it adds, it needs a licensing agreement and access to the car, which can be problematic. PD shouldn't be flying over the moon to get a 250 GTO into the game when there's another perfectly good car to be added at less cost. PD should listen to the fans, but not let them dictate their content. It would be completely inefficient to do so. PD is doing what they can to get as many cars as possible into GT6, in contact with manufacturers and contacting owners, doing what they can.
Also, democracy isn't a beautiful thing: it's the majority taking precedence over the minority. And the majority is just a little scary to me. With such a system, "fame" takes precedence over almost all else, making famous cars more likely to be included. Hidden gems are more likely to be ignored, and that would bother me. I want to see lesser known race cars, such as the Nissan NP35, Jag XJR-14 and Toyota TS010, in GT6, but if the fans chose the cars, these would never make it. It's still slim chances PD includes such cars, but I prefer slim to none.