As someone who used to truck pull, it's because we want clamping force with little to no slippage. Slippage is powerloss, and when you're seperated by fractions of inches, any slippage is the difference between podium and 10th place. But we also built the trucks to take that. A double disc is nice but man they make QUAD discs now that really clamp down with 3 TONS of force or more.