dc motor center rotating magnets
Permanent magnet brushless DC motors usually have the magnets on the rotor and the coils on the stator.

However, a brush motor has a mechanical commutator to keep the Q-axis field vector in the right spot.  If the coils were on the rotor, you would have to bring the DC in, run it through a commutator attached to the shaft, and bring it back out to the coils on the stator.  So commutation is harder.

A PM BLDC motor has electronics to drive and commutate the coils correctly, so there's no extra complexity to having the magnets on the rotor.

Wikipedia has a pretty good graphic showing how the brush commutator works: DC Brush Motor on Wikipedia



By Abnormaal - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4087521