Man, the differences on those rexes are sooo small. And as I remember we already have the JP variant with the darker eyes? These changes are so small it will not even be visible to the gamer, unless you're obsessed with the Rex models.
I don't remember we have any male variants of Brach, Trike or Pteranodon either... Wasn't they just movie skins, which could be either male or female?
And no, quills are not a model change. It's an accessory. You're not another person because you change your hairstyle...
I agree it was weird we got male skins for the raptors, when Frontier states all animals are females. But they are still strongly different than the other raptor skins, so I guess that's why. The rex skins are almost identical, so I just don't see any reason to add them. Especially when it takes work away from important aspects, they could improve on...