meta reason: BioWare is too cowardly/incompetent to try and salvage the setting so they are pressing the reset button hard and fast
That was my first idea back then as well ... but now I think Andromeda was always the plan to follow the trilogy, and does not have much to do with the ending's backlash. Just think of the trilogy'S original intention: A mult-part-storyline that acknowledges all choices and leads to multiple different endings! Yeah, well, we now know how that played out ^^
But think: If Bioware had pulled it through, with stuff like the Rachni-Queen in ME1 really paying off in ME3 (like a giant army of insectoids stopping the Reapers), than any continuation in this Galaxy without retconning or "canonizing" something was impossible. So instead leave the galaxy and start afresh...
I only just began to interest myself again in Mass Effect and Andromeda, so do now know all details revealed by now. I guess the Milky-Way species left either because what happened after ME3 (green bullshte, blue garbage, red nonsense etc), or generation-ships/archs left BEFORE the REapers came, as a backup-plan to save the species?
I hope its the second case, then nobody will have any information about red/blue/green or THE SHEPARD. Might make the game acceptable to play for me ^^