Why isn't the U-2 used for commercial purposes?
It is expensive to operate, and there is no economic value in what it does.

Apart from reconnaissance and espionage, the U-2 is used for high-altitude atmospheric research, but since it is operated by NASA and man-carrying, it is subject to an immense bureaucracy. When you want to fly your own scientific instrument on it, you need to go through years of certification procedures. The instruments which are rated to fly on the U-2 are ancient and much less precise than what can be built today. Therefore, most high altitude research is done by balloons, where the bureaucracy is much less suffocating.