Suspension of disbelief.
The Mass Effect universe is vast, rich, and intricate- and one of the many things that makes it so is the sheer abundance of alien races and species that the player can run into and interact with over the course of the games. Organic, cybernetic, and everything in between is represented in the vast wealth of alien life in Mass Effect games.
And it seems like Bioware are working on something to bring each of these races back to Andromeda. Mass Effect Andromeda is set in the Andromeda galaxy, so far away in space and time from the Milky Way galaxy that was the site of the events in the original trilogy- it wouldn’t make much sense, then, if Asari and Krogans were running around the place there.
“We have a plan, I can tell you that much,” Bioware’s Mac Walters told Eurogamer in an interview. “We have a whole backstory around how it is possible any major species you could have seen in the Milky Way could one day be a part of the Andromeda adventure. And that plan will be revealed when we ship the game. As a small hint to that too, we have one of the novels giving some added backstory to that plan.”
While I suppose it is good that Bioware are trying to approach this problem logically to not break the suspension of disbelief, I just hope they don’t make it too contrived by coming up with justifications for why every major species from the Milky Way ended up in Andromeda. That would probably break immersion far more than anything else would. Mass Effect Andromeda will launch in Spring 2017 on PS4, Xbox One, and PC.