Why are induction motors controlled with a constant V/f?
For an AC machine, the air gap flux is proportional to voltage and inversely proportional to frequency.  (Note that \$\frac{V}{Hz}\$ can be converted to \$\frac{V}{rad/sec}\$ which is equivalent to \$V\cdot s\$, or Volt-seconds.  \$1 V\cdot s\$ is equivalent to 1 Weber, which is the unit for magnetic flux.)  

So, if you maintain a constant ratio of voltage to speed, you maintain a constant air gap flux.  Because torque is proportional to air gap flux, by maintaining a constant \$\frac{V}{Hz}\$ ratio, you can essentially make torque independent of speed in an AC motor.  So you can maintain constant torque down to very low speeds.  As Andy aka said, if you only reduce the frequency to change speed, your flux increases and you end up saturating the steel and you aren't able to maintain constant torque.