Its pretty simple they are a business, so their main goal is to make as much money as possible with paying customers like you. Letting you advance too fast is gonna jeopardize that goal. This system was invented by Blizzard all those years ago and every MMO has adopted it since its really brilliant:
You guys are like rats in a wheel, always trying to get better and even better gear. But when you have managed to get the best gear available you will have no time to enjoy it, because they (BW) simply install an update that introduces new gear, so the rats keep spinning the wheel.
I think the system was invented by Origin Systems, not Blizzard.