From what I have read, it is because our shoes are unclean and because the soles-

[She flips her own shoes after she finishes changing into her own bowling shoes, looking at the grooved underside that she'd never really paid much attention to until now.]

-are not made to help you bowl. There is always the possibility that you may be injured by your shoes sticking to the floor.

[She notices the grimace.]

Is something the matter?