I'm guessing the reliability of the message transfer, and the consistency of its contents declined with the increase of distance. It's like the kids game where

I'm guessing the reliability of the message transfer, and the consistency of its contents declined with the increase of distance. It's like the kids game where you whisper something into someone's ear, which is passed along the same way along the line, and you find the end-receiver wound up with a completely different message. That's probably why they sent our messengers with wax-sealed letters. If they were to build and use such an important infrastructure, it would surely mainly be used for military/political purposes. The criticality of those messages would inherently demand that the message would be delivered in an unaltered state.