The reason for the shift to Andromeda is almost certainly so that the devs wouldn't have to address the endings to Mass Effect 3, and rehash the whole ending controversy. They'll probably have the Ark set out either before or during the Reaper War, and the crew will no longer be in the Milky Way by the time the Crucible fires.
This. This is so obvious it is almost painful to watch people not grasp how obvious it is.
OP, they are moving to Andromeda to avoid thr ME3 endings.
Now, do I think they *should* have made this decision? I realize that wasn't actually your question, but yes, I do. They screwed up ME3 so bad that there really wasn't a reasonable choice to continuing in the Milky Way without choosing a canon ending or homogenizing the endings.