Here's a random thought:
As I understand it, the modified class is supposed to reward those drivers with the best car control and (especially) throttle control skills. If a very hard spec tire having poor grip is selected, then the lap times will be slower, and there will be no question that everyone has enough horsepower. Carried to it logical conclusion, then, all races for modified class should be conducted on ice.
Reductio ad absurdum.
I'll go take my pills now.