I keep seeing people trying to use the story, as well as the fact that this is only one cluster, to attempt to justify the lack of alien races we encounter.
While I understand why one would do that, this is actually incorrect in response to my question because my question is talking about the motivation behind the story rather than the story itself.
The reason it is looking at the motivation is because the story is fake. It could have been almost anything they wanted it to be which means that the story itself is inconsequential. As I pointed out before, if I wanted to have less races in my new story than I did in the other one, of course I could come up with a feasible reason for it, but my motivation remains that I was being lazy.
It's like when they didn't make Vivienne able to be romanced in Dragon Age: Inquisition, and their argument was something to the affect of "When you see her story you'll understand." This was an unacceptable argument because her story would have been whatever they wanted it to be which means she wasn't able to be romanced because they didn't want to allow her to be.
My question was also incorrect because it failed to look at the other possibility that there could have been budget (and or other limiting factors) constraints that caused us to have so much less sentient races featured in this game.
Even with the story they have in place, there could have been 1 or 2 more races without too many changes, but the point is, the story could have been anything. 1 cluster or not, that has nothing to do with whether or not we could have encountered more races because if they wanted us to encounter more (exceptions aside) no one would have complained about it granted there are 100 planets there and all of this is made up.
Granted the number of problems this game already has, I'm thinking this may have occurred due to real world limitations. Adding just one more race to the mix would have increased their workload drastically and thus provided more that could have went wrong. This would be especially true if any romance options would have been available because of this other race.
That's my opinion anyway.