I think it's more of that they want to maintain one type of environment on board a Starship. It always struck me as insane and TNG when they had an alien Starfleet officer who needed some sort of Medical Aid to work with humans in an Earth-like environment.Well, rather than we fill the 23rd Century show with Aliens and diversity of species, I prefer the 23rd century as more racism and suspicious era toward different species in the galaxy. I think everything need a progress. When the 24th Century can be considered as the modern era in Star Trek, where humans and different species are interconnected and live in harmony, the 23rd century should be an era of progression. Where racism, suspicious, hatred still engulf the heart of many species in the galaxy. But the trend goes into positive way like in the 24th century. It should be better or more positive than the bleak idea like in Liu Cixin's Three Body Problem.
But the trend is currently disturbed, because of The war between Federation and Klingon that happen in Discovery season 1. The war really devastated Federation; specially humans in both Military and Economy power. It completely frighten the people.
In a situation like this, there should be a trouble of disunity inside the Federation. Some members would be thinking that join the Federation is a wrong choice. While the others use this moment to question about the competency of Starfleet, and demand that incompetent human to relinquish their leadership to another species. The bickering cause disunity, and disunity disturb a lot of cooperation programs that the Federation has before.
That's why Officer Exchange Program is dead in the era of TOS, and we saw only Spock as the only Alien in the ship. We can say that Starfleet is no longer the trend for Non Humans Federation members as their career choice. As they think that Starfleet is weak and useless.
Spock is half human half Vulcan so one might say that in Starfleet's eyes; he could serve on either a Human crewed ship or a Vulcan crewed ship.
I think the example presented in TOS S2 The Immunity Syndrome made the most sense. The various races of the Federation provide full crews of their own species to man Federation atarships.
