Originally Posted by
Scrodzilla
Calv is right on all counts but the most important reason is vibration dampening. Think of a tuning fork made of aluminum:
designed to resonate a note and ring for a pretty long time. If you were to make a tuning fork out of carbon, it would just "clunk" when you hit it and have very little vibration and as a result, no resonating sound. Now translate that same thing to a bike fork.
i think this is only true to a degree. while its true a aluminum fork will 'sing' over bumps --aluminum frames do too-- i think if the fork is shaped right* the vibrations can be moderated. my 1990 cdale is all aluminum frame/fork and is really comfortable. the fork barely 'sings' over the worst terrains.
*i cannot tell you what 'shape' is right, im no engineer. i'd imagine not much effort is put into building a smooth aluminum fork nowadays when a carbon fork could be had.