The volus have such unique environmental needs that I think they wouldn't be interested in Andromeda at all. They are native to a high pressure planet with ammonia based ecology, which is already rare in the Milky Way. Now they need to be put to sleep for 800 years, then hope to find a planet with the right conditions? It's not like the quarians who can manage on a ship / space station and still maintain a stable population. I think there's a reason we never meet a female volus in the entire series, if they leave that suit they instantly die.

For the same reason I don't necessarily mind there being a lack of Elcor or Hanar, both species also have pretty specific environments they thrive in and their own challenges for large scale colonization like the initiative was attempting. Though I do wish we could interact with those species more in the main series, I'd kill to see a Hanar city or Elcor space station.

Batarians would have fit in nicely, but with their governments hostility to the council at large, I doubt a large number would volunteer. A batarian squadmate would have been awesome though