Why are potentiometers not a good method of speed control(in terms of energy)?
there will be huge loss in terms of energy thereby decreasing the efficiency of the motor. for example consider 200V, 1HP motor. if you decrease the voltage by potentiometer current will increase for same HP...which increased I^2R loss...you cannot increase the voltage beyond rating because this will cause insulation problem and flux weakening...
Aside from the obvious wasted power, speed regulation is poor.
and also finding ones that have any significant power handling capibility is difficult and expensive
Scalextric speed controllers are a cheap and cheerful example but, because of the short lap times involved, the bad regulation doesn't matter and the player can compensate with skill.
Speed control of large electric locomotives used to be achieved partly with series resistors. Those resistors were fearsome looking but I think the resistances were switched with a large brass wiper and brass contacts, rather than constantly variable.
Separate names with a comma.