The game starts 16 some years after EPIII. At this point Yoda is assumed to be on Dagobah, a deleted EPIII scene shows him going there (which will probably eventually be added to the movie) and it probably happened right after obi-wan and him spilt in the hopes of giving no time to track him down. So yea there is really no reason of having Dagobah that couldn't wait till an expansion. Delphi mentioned something about it being a good hidden base, but really when you go there you want to see Yoda. And I know I mentioned yoda wouldn't put up with imps on his planet, but also if Vader, The Emperor, or Obi-wan were there they would be able to sense him in a heartbeat.