Originally Posted by Curt Kurt
Becuase you can modulate just as well and even better!(with a lot of practice) with your pedals than you could with your rear brake.
Not really.
With a brake you can apply a constant force to rim(or rotor) through the pads. Unless there is something wrong with your braking system this leads to constant friction and a constant amount of tourque on the rear wheel.
Applying force to the pedals in either direction you can't do this. You are using different muscles with different strengths throughtout the pedal stroke and therefore you can apply a variable tourque to the wheel. Unless you are riding a really really low ratio or are insanely strong you probably are applying a variable stopping force through the back wheel which makes you much more likely to skid. Think about forward pedaling. People who spin out while climbing almost always do it in the middle of the pedal stroke and then regain traction when the pedals reach the top and bottom.