Imagine the transcendental's message space as a canyon. It's narrow at one end (when they are born) and widens towards the other (in the direction of increasing time).
Each message sent back in time is a string tied down the canyon, from its destination up to its origination point. The widening canyon means more ability to send messages, but not more ability to send them back farther.
A message is never, ever removed. Once you've decided to send it, it stays tied in place forever. In this way, the Transcendentals are limited in how much information they can send back to a particular time and before it. Obviously they can send as much information forwards as they want.
Most of the canyon's width is taken up with three things, from largest to smallest:
1. Communications that comprise the T's sentience.
2. Instructions on how to build more temporal bandwidth.
3. Information on the Desired Future.
Mission goals and details for the Investigators are part of #3. #1 is larger by orders of magnitude.
Because of the information sent in #3, the canyon is optimally packed with strings when it comes to reaching the Desired Future.
Does this analogy help?