Please Give me reason of this (Single phase induction )

Why should it?

The currents induced in the rotor are distributed around the many rotor bars. Because the magnetic field in a single phase induction motor is not 'rotating', it works out that the distribution of current in the bars ends up with half the bars having a force (current carrying conductor in magnetic field) that tries to turn clockwise, but just as many have a force and position to try to turn it counter-clockwise and the net torque adds up to zero.

daestrom