Talking about the expense factor, seems a bit odd if just concerning the EU, since a majority of that is simply comic books and novels. The games mainly do the same thing as the movies do, have humans at the forefront of the story. But for games, you don't need to spend money on either make-up or prosthetics for an actor, only to model the creature and give it a voice(which for some types of creatures may be rather taxing and complicated, but still do-able).
But as to the relation aspect, it is of coruse easier to relate to a character that is human, than it would be to relate to a Twi'lek, for instance. But then, depending on the story of the character, it is possible to relate to them. Part of a story has say a Twi'lek, Zeltron, or Chiss on a prejudiced human world? Easy to relate to if you understand how prejudice works, or have experienced it(which is unfortunate).
Im just wishing that if they don't use the alien characters in the movies, then they should at least include some as the main characters of games. Sure ones like KOTOR have the aliens in the game, but not as main characters without mods. JK:JA is one of the few games where it is possible for the player to have an alien character appearance or avatar by default. And in a way, that would be all im asking for. Well, that and to make some of these characters have canon appearances that aren't limited to male humans.