I have a feeling that the initital work done on the Alien modules reflects the original Imperium game in which there were races such as the Vargr, Aslan, Centaurs, Hive, Dorsai and regular humans.
Some work was done to elabourate the Centaurs but they did not want them to appear too Alien, as seen in the Alien Handout...similarly the Hivers remain how to make things more Alien. The K'kree were the first attempt at that. As both AM 1&2, state that aliens will be more than creatures in a rubber zipper suit. Having done the Catman & Dogman archetypes common in most 1950s Science Fiction, they sought to do something different with K'kree and only got a half-way result (the hand being the most interesting feature of the K'kree & the extended herd structure).
Then they moved the Alien within us or the Alien as an outsider...which fits in the Zhodani mould. However, there were other considerations, such as the FFW had just introduced the major enemy of the Imperium as the Zhodani. However, the point was not to show them as evil but simply misunderstood.
The Droyne come right on the heels of the Secret of the Ancients giving some sort of backstory to the trilogy (who at that time really knew what a Chriper was, other than the cute alien RS Gamma). Giving the Droyne a culture and history fitted together with the AM.
The Solomani was probably in response to the release of the Boxed Game...the Solomani Rim War and also Traveller players asking what happened to Earth in the Far, Far Future...a question still needing a better answer...
And, lastly the Hivers were the last to get treatment because they had to be something really weird, as Traveller had not escaped SF archetypes all that well.
But, for the like of me, I cannot explain the popularity of the Darrians...maybe they had a stray manuscript from the Spinward Marches Campaign and wanted to put it all together with a Sword Worlds supplement dealing with the Small Cultures/Races of the Imperium...
In the meantime, MT happened and DGP began to rewrite over some of the cannon and begin to give their take on the Alien Races and make them appear more alien. Sadly, RS has kept those treasures away from view.