It's political to have a diverse crew, so the more your crew is seen by outsiders who will Judge the Federation as maybe a humans only club, the more diverse the crew has to be, not to seem like space hypocrites as they build a space brotherhood.
As a science vessel, Equinox was sciencing, not meeting aliens or fighting wars, so there was no need to pretend that everyone in the Federation is equal, and create a rainbow crew representing all the member worlds, so thanbkfully a human crew doesn't have to put up with weird alien behaviour.
Besides?
Are we sure that the XO, who transmogrifies into a pillar of black smoke, now and then, is completely human?
Although...
(Simple) AI would monitor for a racist crew selection?
When ransom was filling out the forms to make his crew, the computer probably spat back at him, too many humans, pick something else, like how when you try and invent a new password for you email. and the computer laughs at you "Please use at lease one capitalized letter, one numeral and one non-alpha numeric character."
Those %100 Vulcan ship seemed racist, but they were not racist, they just don't want to have to take constant painful injections to hide the fact that all non Vulcans smell like garbage, which seems fair?