You're comparing apples to oranges. One whole galaxy linked together with mass relays, to one cluster with no way to connect to other clusters.
Datapads make references to other races outside of the cluster, the kett make references to the countless other species they've worked into their genetic structure.
So it has nothing to do with the milky way being special, or bioware being lazy.
What Arsenic above said!
Plus, if you read some of the planet entries, they mention advance species that are extinct. Sure it's not a great addition to his points above, but it's just more proof there are/were more other races out there.
Considering how the Kett are divided though across the Galaxy (presumably) it's likely there will be less diversity anyway, because they will have annihilated some species of interest.
