I think it came down to too many choices. If Shepard lives he or she might have retired. If Shepard dies then he or she is remembered for their sacrifice. However the control or synthesis endings make no sense. How would they even explain the forced merging of organic and synthetic life. Or that Shepard enslaved the reapers then enslaved the galaxy.
The put too many ending options.. that would be too difficult to summarize in a different game. Then Andromeda begins and it's like guess what ? No one cares how the reaper invasion was resolved.
It was really odd to throw that many options around. In mass effect the ending was this do you save the council and who is the human council member.
In mass effect 2 if everyone doesn't die. The options are destroy or preserve the base. In mass effect 3 we have three options plus the do nothing option if you reject the three options.
Two out of three outright kill Shepard. One option the destroy option Shepard can survive. Control ending changes depending on paragon/renegade scale.
They set this whole thing up be the grand finale. Yet at the time never thought. If we make a new story how do we explain this . Five years later Andromeda is birthed. It completely avoids this topic.
Yet because they tried to loosely connect the two. Which is why the initiative begins as a group leaving the milky way for Andromeda. Instead of being descendents of the first settlers