Where's the logic in having a starship crew that isn't
as diverse as the Federation it belongs to? For that matter, where's the logic in writing a science fiction novel and not taking advantage of the freedom of the prose form to depict more exotic aliens than you could on television?
In-universe, the reason for the push for diversity in the Luna
class is much the same as the reason for the repeal of "Don't Ask, Don't Tell" -- to increase the fairness of Starfleet personnel assignments and to ensure that the ships weren't missing out on good, capable crewpersons simply because the ships weren't designed to accommodate nonhumanoids. Again, it's more that there's no legitimate reason not
to allow as much diversity as actually exists among the population. It also serves the purpose of demonstrating to newly contacted races that the Federation is inclusive and open, which can promote good relations.