Why is easy Hunter they do not work in the real world and the gas prices are still to low.
To date we do not possess any technology that is capable of collecting enough solar energy to push 2000 lbs of metal at 70 mPh up a hill given the current surface are of a modern car. Heck we have not been able to make an electric car that can do it either for any great distance with out a recharge. Thus the evolution of the hybrid which is still not good enough to justify the extra cost based on the price of a gallon of oil in Canada.
The technology is getting better, but lets face some basic thoughts. The technology is controlled by the oil barons who own all the patents and suppress any technology that might actually have great potential.
Once they milk all they can and get the public to accept the idea of solar cars or pure electric cars then and only then will they start mass production of the cleaner and better ideas, but that is 20 to 40 years down the road.