The problem is that the rotor and stator must have spaces called slots, in the iron for the conductors. The conductors and air in the slots have a lower reluctance than the reluctance of the iron. There are more slots than there are poles in the stator. The windings are distributed among the slots to form distributed poles. Their is a different numbers of slots used for the stator and rotor. Those two features prevent the rotor from becoming locked or "cogged" in the minimum reluctance position that would otherwise exist.
Even with proper selection of the number of stator and rotor slots, there can be reluctance variations at different rotor angles that cause torque fluctuations and inflections in the torque vs. speed curve. Skewing the rotor slots mitigates that effect.
Here is a picture of the rotor and stator of a 4-pole, permanent-split capacitor, single-phase induction motor.