The reasoning behind this is simple: it’s safer.
When your child is strapped into the back seat of the car, he or she is one third less likely to die in an accident. This is particularly true of head on collisions, where children in the front seat are usually thrown through the windshield, or onto the dash board.
If your car has air bags, this is even more important. The force with which your air bags deploy can injure or even kill your child, because he or she is shorter than an adult, and therefore not positioned correctly for the airbags to be safe.
Even if your child is in a rear facing car seat in the front seat (the safest car seat, but still not suited to the front of the car) he or she is at risk. That’s because the back of the seat is so near the dash board, where the airbags are. If you do have to have your child in a car seat in the front seat, then try to make sure it’s in a front facing seat, and that it’s the child that is least likely to attempt to wiggle out of the seat. Find out whether there’s a switch to disable your airbags, and use it. If your child is no longer in a car seat, and you have more than one child to transport, then choose the biggest and tallest of them to sit in the front. Don’t use a rear-facing seat in the front seat without first disabling the passenger airbag.
Essentially, this is something you should try to avoid if at all possible. It’s just safer for your child to be in the back seat.