Because, basically the PR tires are only good for street use on warm days with clean dry road surface and properly preheated riding before "letting loose". They are the ultimate for hard core grip but only for a surgically appropriate environment. There are at least 5 different high performance street tires that are a far better choice...PP 2ct, Corsas, Bridgestones, etc.
The specific inadequacies are...longevity, lack of grip in corners with debris/ water/cold surface, heat cycle situation(ok for tracks, not for street), low psi requirements for performance(compared to street tires) highway vrs. twisties needs differing pressure.....I could go on, I just think it's important that we are all learned about these realities....ultimately it's about safety AND performance and hi po street tires just "pack the gear" better for street. In short, track tires are an "accident waiting to happen" on the street and the "ultimate" on the track.
Raydog