I heard a theory that these are people specifically picked and put on the plane for a specific reason. The Hanso Co. conducts experiments and this group is their present experiment group. The people they keep finding that are dead are from previous experiment groups. Apparently the Canadian promo said they found a weird list with everyones name on it.
If you go to www.oceanicflight815.com and click on theory board on the bottom, it gives you a bunch of thought theories.