During the PTS one of the biggest pieces of feedback surrounding the Expeditions was that the challenge and rewards didn't really feel good enough or worth it. In part off of that feedback, we decided to hold off on introducing the Mastery System in order to better hone and improve the experience and rewards to be more... well... rewarding.
We have a more official article up discussing the Expeditions here
if you're looking for some general information. But to be rather blunt, based on the feedback we were receiving during the PTS, the Mastery System wasn't up to snuff and wasn't going to be ready in time for TU5.
As the above article says, we'll be sure to share updates when we have more info to share.