You can't keep them all. People do move on to other things. As much as we, the fans, would want certain people on this production, I'm sure the producers would like to have certain people on board as well. But people move on to other media, other work, etc. Then you have to ask if their schedule is open for the time you need them. People like Miyatake or Itano may be called in later to overlook certain elements, but they do have other things to do BESIDES Macross.