Because the shooting world has finally realized that the 40 S&W was the answer to a question that never needed to be asked. We can't give them away at the shop anymore.
You are correct. It was developed basically as a face saver for the FBI when their 10MM concept fell flat for law enforcement. The 10MM is a great caliber, but too large and too much recoil for many current FBI agents.
Law enforcement jumped on the band wagon just because it came about due to the FBI and their adopting it. The problem is that it is just too "snappy" and will do little that a properly loaded 9MM or .45 will do.
I see them in local gun shops about 10 to 1 over 9MM and 45 third generation S&W's. They are also priced much lower than comparable guns in other calibers.