Because the build bar can only have so many units on it at once.

You complete the little side quests and they will be awarded. I think you get 4 or 5 platoons and they can be any legion/battalion depending on which one you have selected when you deploy them. For example, if you want the Galactic Marines, you need to have the 21st selected or have Ki-adi-Mundi or Baccara deployed (idk how to spell their names)