I think one reason front kicks and side kicks (my personal favorite) aren't used as often is similar to what someone above mentioned already - most people trained primarily in roundhouse kicks and probably aren't very good at fronts and sides.
I agree with YouKnowWho's spear analogy, which is why I like the side kick so much - the last few sparring matches I've participated in with headgear and cup I've been able to land about 4 or 5 of these and leave just one or two footprints - yay for accuracy! But I would say few people have the flexibility to control the side kick as well as they should, to be able to land them on a consistent basis.
The second thing about the front and side kicks is that they do tend to be pretty easy to catch... if you meet the right (or wrong) person doing them. With these kicks, it is almost as important, if not moreso, to be quick to chamber as it is to be quick to deliver. Quick to deliver is obvious - force = mass x acceleration. But quick to re-chamber prevents your leg from getting caught. Flexibility also plays a big part here - more flexible guys (and gals) can retract their leg on the same trajectory that they attacked, providing less space to catch. Guys that get caught often simply fall into the opponent's grasp, even if they landed the kick, because they failed to retract quickly enough.
That said, I still love me some roundhouse kicks. My sifu used these even more than his hook kick (which his accuracy with still bewilders me to this day... I think I could literally run and hide in the bathroom and still get hit in the head by that kick), targeting the thighs and feet to trip you up or otherwise impair your ability to move.