Because they shouldn’t do it.
It doesn’t make any sense to the story; regardless of faction choice.
Alliance still has Warlocks within it’s ranks and Demon Hunters.
It’s not clear because it didn’t happen.
The Highborne of Suramar fought to ensure the safety of Suramar’s inner city and it’s Temple. At this point, the Night Elves of the Resistance had already began their march onto Zin-Azshari from Val’Sharah.
Elisande chose not to make contact with the Resistance as she didn’t believe it would succeed.
That’s true, but the only Night Elves who the Nightborne had contact with were the Moon Guard and they are not of the Alliance and the only Nightborne who likely met the Moon guard were exiles, who would later become Withered.
Change happened to the Northern Kalimdor Kaldorei and they were difference from their Moon Guard kin.
Aside from that, the Nightborne kept themselves to themselves
Night Elves who were neutral, not apart of Darnassus.
The Nightborne have absolutely no relation to Darnassus. It’s why trying to force the Nightborne to feel anything for Teldrassil is nonsense. They have no connections to any world tree of age, let alone the youngest of the world trees.
And that is why the Nightborne shouldn’t be there.
I’m more happy that they joined the Horde > Alliance.
Making them “Humans and friends” is lore regression.
Certainly, but we don’t know their allegiances at the current time. They might be in Stormwind, opening a portal to Azsuna, but that’s neither here nor there.
I would, but it isn’t just night elves. It’s the whole Alliance!
That’s regression, not progression.
The Nightborne have a chance to progress with the Horde, whilst still keeping their thing and they do. Portal Magic, Leyline Knowledge, Arcane Barriers and Telemancy have remained their key attributes before and after joining the Horde.
They have become the Magi of the Horde, which is great because Blood Elf Mage lore is history as they are very keen on keeping the blood elf lore tied to “Blood Knight” lore. Can’t blame them though, since they are hesitent to expand Rommath’s character and decide that Blood Elf Mage lore needs to be about Aethash!t.
Legion told us that it’s not always about hating the other faction. The factions unite to fight common evils.
With the announcement that the Night Elves and Nightborne will be present for 8.2, I’d say they will work together, but the latter will simply not abandon the Horde. That won’t happen.
We don’t have concrete proof that any possible relatives between Darnassus and Suramar are there. Yes, we have Elves who came from Suramar, but no mention of relatives between the two.