Why don't all new GA planes include a parachute system?
Airframe parachutes are becoming an increasingly popular option on newly-certificated aircraft, thanks in part to the track record of successful deployments on Cirrus aircraft.  

If I had to speculate on why they're not showing up in older certificated designs (like the Piper PA-28 or the Cessna 172 &amp; 182) I'd go with the reason I mentioned in my comment: "New" Pipers and Cessnas are basically extended manufacturing runs of very old designs - going on 50+ years. Changing an existing aircraft design to include the required hardpoints, breakaway panels, etc. for a parachute system would be a substantial change to the type certificate, and under Part 23 certification rules doing so would cost the manufacturer an equally-substantial sum of money for the design, engineering, and testing required.
Light GA aircraft sales at the moment are, shall we say, lackluster -- the additional costs for the engineering, the type certificate update, and the parachute itself would certainly not help matters, as that cost would need to be passed along to the eventual purchaser of parachute-equipped aircraft (which would probably mean putting a 50+ year old classic Piper or Cessna design in the same price category as a clean-sheet Cirrus SR20/SR22).

In addition to the mainly-financial reasons above Quantas 94 Heavy has a point regarding weight.  BRS (the premier airframe parachute company) makes a STC'd kit for some Cessna aircraft.
This kit costs about 80 pounds of useful load, and on many light GA aircraft the useful load is marginal enough that this added weight means you're effectively sacrificing a passenger to the installation. The Cessna retrofit kit also takes up a substantial amount of space in the baggage compartment as you can see from their installation white paper, which is certainly a negative factor as well. Were the chute to be incorporated into the design by Cessna it would likely be installed in a similar location (there just aren't many other places to put it) with a similar cost in weight and cargo space.