The reason is basically that this is a beta. The actual service won't be GA until next year. During this period, Google is likely wanting to control the hardware, to reduce issues, and give it time to ramp.up it's support infrastructure.
The long and short is that you're getting a chance to try it out early by buying the Founders or Premiere editions. If you wait until it officially launches to all, along with the free tier, then you can use your own hardware and potentially.wont have to pay for anything but the games you want to play.
