Why don't most MTBs have rear shocks? This keeps me wondering. I normally see front and rear shocks on downhill bikes, but when it comes to touring, road, or mountain bikes, almost every one has only the front shock.

What is the reason for this?
Price - a suspended frame is much more complicated and expensive to make, and a good shock alone can cost more than a decent rigid frame.
Maintenance - both shocks (due to gaskets/seals, lubrication, cleaning) and elements suspended frames (additional bearings) are relatively high-maintenance and short-lived, compared to rigid frames. This also translates into less reliability, especially important in touring/trekking trips.
Cost-effectiveness and weight-effectiveness - especially in road biking or touring, it's not such a big deal. Much better to put wider tires or suspended saddle.
Pedalling efficiency - even the best suspension systems rob some of the user's energy. In mountain biking, it's more than offset by less energy expended by the rider, who can spend less time out of the saddle on uneven terrain, but in road cycling it's absolutely unjustifiable.