Induction from AC or RMF
So why does this need to be AC?


Three-phase AC current makes the magnetic field rotate.


  Basically, what causes the induction?


The rotating stator field induces AC voltage across the rotor conductors resulting in AC rotor currents. Those currents produce a magnetic field that rotates with respect to the rotor.

The above is the intuitive explanation. The motor can also be viewed as a device that has an equivalent circuit that is a transformer