[Al Skierkiewicz]

OK,
Here's the real reason. The speed of a motor is directly proportional to the current flowing through it and the load placed on the output. For a fixed input voltage and a fixed output load the current will remain constant. For a fixed input voltage, as the load becomes greater the current will increase. Now with that said, the motor winding is essentially a resistor, so using Ohm's Law, if you have a fixed resistor value and a fixed voltage source, then the current will also remain fixed. The only way to force more current through a motor winding is to increase the voltage.
Now comes the hard part. The motor has other losses, frictional, electrical and magnetic, that require a certain amount of current to overcome. Once those initial losses are overcome than the motor will speed up even if you hold the voltage constant.
The one thing that speed controllers do is to switch on and off the full supply voltage from the power supply, and vary the length of time the voltage is applied to the motor. This high voltage overcomes the losses (especially at low RPM) so the motor has greater torque and more reliable speed regulation at low speeds. The length of time that the controller is turned on determines that average current flowing through the motor. This output voltage switching is commonly referred to as PWM, Pulse Width Modulation. When the "ON" pulse is very short, a small current flows through the motor and when the pulse is very long, a large amount of current flows through the motor.
If anyone still has a hard time understanding this, please PM me and I will try to answer this better for you.
The simple answer is, Yes speed controllers are far superior to variable voltage sources.