Kishi never gives us all the answers in one go, that's the case for years now.
A lot of characters and their stories are linked, so it's impossible to answer a question without spoiling the entire mystery. For example, revealing what happened to Shisui would be a dead give-away to Itachi being a good guy all along.
Secondly, he also holds back answers so that it won't potentially ruin a battle. Nagato and Itachi's backstories were revealed after the fights were over, so that it wouldn't destroy the suspense or make the endings too obvious. If we had found out Itachi was good before his fight with Sasuke, we would already know Sasuke will walk out unharmed no matter what, which would make the entire fight pointless. But hearing the truth afterwards made that battle a lot more tragic, and perhaps a little better.
Who knows, maybe the full explanation about Madara and Obito will shed a completely new light on the current battle, in a good or bad way.