Because the one guy that tried it was running away from the field so they outlawed them. Of they would have to worry about turbine's holding up under the extreme combustion temps of nitromethane as well.

Anyway, at a given boost level, a turbo will make more HP than a supercharger. Period. Now if you get a positive displacement supercharger setup, you may make torque sooner than the turbo, but for HP, turbo is king. And its GREAT to be the king