The dimmer you had was probably a cheap dimmer that uses resistors (or something) to reduce the voltage to the motor. If it's a 100v motor, it needs close to that to operate. If you drop the voltage to 90v, it might spin slower but at a certain point there won't be enough voltage to make the motor spin. So 50v might seem like it will reduce the speed by 50% but in practice it won't spin at all.
A fan control switch probably uses PWM (pulse width modulation) and will be more expensive. PWM supplies the same voltage but it does it in pulses. So instead of always having electricity, it will switch the voltage on and off really fast. It does it very quickly so that the motor turns on and off many, many times a second. It gives you the full voltage so the motor will spin but the motor won't get to full speed because it doesn't have the voltage on long enough.
If you are really interested in how it works, read this: http://en.wikipedia.org/wiki/Pulse-width_modulation
Long post short -- buy a more expensive dimmer and it will probably work.