It's also I think disrespectful to the rarity of any alien life in the real world to overpopulate Trek's fictional one with WASPy foreheads-of-the-week. It's a matter of suspension-of-disbelief.
Of course, the Screen Actors Guild only supplies WASPy ones in the most plentiful supply. I believe GR once also said something along the lines of,
If an alien is going to be delivering lines, then I want to see the actor's eyes.
One of the strengths of the first few arcs of the LA Times post-TMP comic strip - and the first Marvel comic series (set in the same period) - was that the great array of "new" Federation races barely glimpsed in TMP were being featured as guest characters, and populating background scenes.
Of course, the reason we never saw them talking
on-screen was that most were over-the-head latex masks (similar to DS9's Morn, but not nearly as sophisticated - yet) and the actors couldn't talk in them. Hence the Rhaandarite ensign - the most humanoid of the lot - was the one to get the one line of dialog on film. I remember Fred Phillips saying that when he started making all the alien masks, GR had planned there'd be a banquet scene, similar to "Journey to Babel", and that the bantering ambassadors would be interrupted to be informed about the mysterious cloud heading to Earth.
The early Pocket novels (and ST II) missed the boat by not exploring them, or at lest revisiting them. Many years later, Christopher Bennett's "Ex Machina", being a novel without makeup restrictions, and a direct follow-up to TMP, did a great job in reminding everyone who these races were, how fascinating they could be - and it's great that many specimens of these species have secured themselves prominent places in other novel series now.