Geek question for someone who hasn't read the entire trilogy: what were the Sheliak, Tzenkethi, Miradorn, Jarada, Tamarians, or Dominion doing during the Borg invasion?
In addition to the answer above, we don't know what shape the Dominion is in 5 years from "now."
And the Jarada are... jerks. I can't see them helping out. They'd attack the Borg if they approached the Hives, but they're so... what was that term in the DS9R? Zeno-disdainful or something? I'm not convinced it'd matter to them who was being attacked, as long as nothing bothered them...