They're used because they're cheap.
I understand it as mostly having to do with the way the armature is wound and mechanical forces; salient-pole motors, with the coils simply just wrapped in loops around the rotor like it's a rotating bar magnet, are not practical for high speeds, but are cheap to manufacture. The magnetic field offered by a cheap permanant magnet inherently limits the power and speed possible, too, but is cheaper to manufacture for small motors than other types of cores.
I think you are thinking of a Shaded Pole motor. PSC stands for Permanent Split Capacitor. They are NOT necessarily cheaper than Cap-Start motors and have less starting torque, but they have the advantage of no moving parts (other than the rotor of course).
