Put simply, the rewards program is a short term marketing ploy to get you into the store and purchase. By limiting the time you can roll over your points, they hope to get you into the store more frequently. They really don't want to see customers roll the points over for big purchases. Something you could always do with some other programs in the past.
Still remember a while back, my one friend walked into the CTC with a breifcase full of funny money to buy a canoe. The person at the service desk almost fainted.