It's just that without Keldeo, it would feel awkward and out of place if it wasn't in the movie and in another movie where the three Trios not in it.
It's possible that Keldeo could presumably be in this movie (maybe they just didn't show it because they don't want to officially reveal it yet, perhaps Keldeo will be revealed through some other method first then the trailer will be expanded, after all we do have a year to see)
I do believe that Keldeo and/or Genesect could get their own movie, after all they like to get as much out of each legendary pokemon as possible. The trailer and title for this movie is very specific so far as to only the 3 of them though.