Mainly for following two reasons
1.To avoid core/eddy current losses in the magnet.
2.To avoid generation of induced emf in the objects being lifted.
Well you'd rather have a constant magnetic field than an alternating one. That way you get the most average power. In a sine wave, the average power is only half of the peak power so you're wasting half your energy.
With AC ,to obtain the required mmf ,more voltage and more turns are necessitated due to impedance and back emf
