1) The pilot in command of the aircraft is responsible for ensuring that anybody on the aircraft with a parachute is capable of operating it safely. This is unnecessarly burdensome for commercial operations.
2) Parachutes are not something that just anyone could put on properly. Putting on a parachute improperly can cause serious injury or death.
3) Jumping from an airliner at crusing altitude requires supplimetal oxygen, which requires special training.
4) Jumping from ANY altitude requires special training for that matter. A normal ground school is 6 to 8 hours.
5) Parachutes are expensive. Mine cost 1200GPB (about 2500USD) and that was nearly 15 years ago. Supplying one for every passenger on an airplane would be prohibitive (especially with supplimetal oxygen).
6) Opening a door on a pressurized airplane is extremely dangerous (even at lower altitudes).
7) They don't make parachutes for children or pregnant women. Also people weighing over about 18 stone would need a special parachute.
8) Jumping over water requires even more training and special equipment.
9) Could you imagine the pandemonium? The plane fills with smoke or starts pitching and rolling severely and then the flight attendant demands your attention to show you how to put on a parachute. Chaos.
The only time that parachutes would actually be preferable is in a situation in which the aircraft is definately going to crash violently, but is currently under control and not filled with smoke or losing pressurization and shouldn't be expected to for some time.
I'd say that you would need a minimum of 30 minutes for a staff of flight attendants to get 200-300 people into rigs and ready to go out the door. That's a very low estimate.
The likelyhood of an aircraft being stable for that length of time, but also known to be doomed to violent failure during landing is too small to consider. In the end, it just isn't worth it.

