I am disappoint wrote...
But the entire plot of ME3 needed to reveal them for it to work.
Of course. What i mean to say is that Bioware didn't need to expand anymore on why the Reapers harvest us after ME2.
Leviathans create Reapers to serve them, Reapers rebel, then invent a way to sustain themselves: the harvest. Simple, end of it. They are like a parasite feeding on evolution and must be stopped.
This said, we are left with two points.
1- Victory should have been conventional-only, with endings branching per your choices during the trilogy.
2- Locating the Leviathans and learning the truth should have been the central theme of Mass 3.
No need for the crucible, the catalyst and the endless ocean of hate they birthed.