The Horde didn't have an ongoing multi year campaign begging for a duplicate of a Horde race for one thing. My educated guess was that they knew they were going to give the Alliance some kind of high elf subrace in the subrace system for the post Legion expansion and they seeded Nightborne to justify why the Horde would have that. And then they decided they couldn't give the Alliance a duplicate of a Horde race so they made them a variant as well.
Which we got at the same time as the Alliance received a high elf model with a skin colour change. What's your point? You got a variant of a Horde race, we got a variant of an Alliance race. You will note Nightborne are NOT the same as Night Elves, just as Void Elves aren't the same as Blood Elves.