Because he has to bring the power to the road ... Is the way in every two cyclists so
Because this is the drive tire, on which the engine transmits its power to the road. If the as narrow as the Front, he would wear too quickly.
If one were to make similar demands to the Near as wide as the rear, the bike would become very restless and can be grauslich steer hard.
btw this is not only for bikes so, with all motorcycles rear wider than front.
I assume you mean wider. But even that is not necessarily like that. The reason is if it is so, the better traction.