It could also have been, of course, "ingress" of all sorts of unmentionable er,
debris! I would expect a
universal motor (that is, a series-wound commutator type) in most low-end mains-powered suction pumps, by the way. But I guess that, in this thread, we are basically talking about simple battery-powered portable suctions units. And therefore, simple DC motors, with commutators and brushes.
Brushless DC motors are around, of course (you find them, for instance, in 3.5" floppy disk drives) ... and they are essentially permanent magnet AC motors. But they require the added "complication" (read as
cost) of the controller circuitry.
But (just in case there's anyone reading who hasn't already noticed), where the brushless DC motor has really "taken off" is in the realm of flying model aircraft (yes, that
was a pun)!