DC motors have effectively one variable: how much power are you feeding the motor? AC motors have two variables: power, and frequency. I'm not an expert in motors, but I'd expect that AC motors would thus allow independent control of speed and torque, while DC motors do not. Directional control is also a concern. An AC motor's direction can be controlled by the rotational direction of the power being fed to it. A DC motor's direction isn't so easily controlled.
More broadly, all motors operate because there's a rotating magnetic field somewhere. That rotation is either generated inside the motor (self-commutating) or because the power feed to the motor is itself rotating (externally commutated). DC motors must be self-commutating; DC is definitionally not rotating.
How do you achieve commutation inside the motor? Typically, either there are brushes, or there's an inverter built into the motor. Brushes wear out, and I suspect have other disadvantages. And if you're going to build an inverter into the motor, why not put it outside the motor and get better control of it?
Universal motors have a low manufacturing cost for the high quantity manufacturers. When the load increases, they slow down and develop more torque without drawing disproportionally high current. They are inefficient, but that doesn't matter very much for things like food mixers and blenders or small power tools that don't require much power and are generally used only occasionally for short periods of time.
With universal motors, variable speed can be implemented inexpensively inside the appliance in a small space.
Permanent-magnet DC motors are more efficient and thus better for battery operation in terms of longer operation between charges. They are used in cordless power tools and appliances. Those that I have examined don't use the most expensive types of magnets.
We may see electronically controlled permanent-magnet motors take over more of the small appliance applications as manufacturers reduce the manufacturing cost for that technology. The Dysan "digital" vacuum cleaner motor is an electronically controlled permanent magnet motor.
Expanded in response to comment:
Universal motors can easily operate at higher speeds than induction motors. That is particularly useful for vacuum cleaners and blenders. When operated from domestic utility power, an induction motor is limited to RPM = 120 X f / p where f is frequency, either 50 or 60 Hz and p is the number of poles, minimum of 2 (or 1 pair). That means that in induction motor can operate at no more than 3000 or 3600 RPM. It is not difficult to exceed for a small universal motor to exceed that. The speed of a small universal motor can be as high as 15,000 to 20,000 RPM.
Best Answer
The letters are "representative of something"
To assist in providing an international standard the International Electrotechnical Commission (IEC) created that IEC 60034-8 standard. "Terminal markings and direction of rotation"
An electrical machine compliant to 60034-8 will ensure that clockwise rotation of the drive shaft will occur for positive electrical phase sequence U-V-W.
Thus if you were to wire such a machine upto a distribution labeled A-B-C and these electrical phases were A leading B, leading C... you would have the expected mechanical rotation
is it "just another set of letters"? you could say they arbitrarily chose that sequence, a sequence that is just as good as any other BUT they assigned a specific meaning to it and a meaning that is internationally recognised.
MotorA from supplier A and MotorB from supplier B being compliant to this reg & meeting the customer requirements would be drop-in replacements that would minimise any unexpected mechanical rotation