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Beloren answered this very well, and let me highlight what resonated with me the most, in his response:
Spot on here
Also this, in particular the bolded part. People may cry and scream about the similarity of the Nightbonre to the blood elves, but there is no denying the Nightborne are showing you kaldorei pre-sundering culture, not Thalassian culture, and this is also the culture of the Darnassian Highborne. You are being shown Kaldorei culture, and that part of it is similar to the Thalassian, because the Thalassians are a culture evolved from that part.
People too easily forget that high elves and night elves are Elves, they have far more in common than not, but the faction divide creates that illusion. And blizzard are all to happy to foster it, so they emphasise the Elune nature side in the night elves, more than the arcane side that holds the commonality with the high elves. But it's there, quite plainly in the lore, and easily evidenced in the Nightborne and Shen'dralar Highborne as being very Kaldorei.
THe night elves are the elf progenitor raec, just like the Zandalari are for the trolls.
Because for the alliance to grow, it has to come at the expense of the horde. The horde growth through the blood elves, took alliance fans over - it was at the expense of the alliance, both in losing the high elves (as playable) and in the stunted development, suppression and nerfing of the night elves so they would be amazing or attractive.
Now the horde has the far greater numbers in the elder game, you need some of that population back. You won't get it back if the horde elves remain so pretty and pristinely radiant on the horde - the most effective way, as Ravenmoon puts it, is to take Sivermoon/Quel'thalas and Suramar from the horde and blow up the high/void elves and night elves so they are very attractive and awesome again. This will lure some of the hardcores back, at least the elf lovers. Inspiring them to make things work on the alliance, and reducing the horde elf fanaticism which is a distortion on the identity and spirit of the horde.
Face it the elves in their pristine advanced culture don't fit with the horde races, but the alliance ones, and now the horde is popular (which is the reason why we had them so amazing on the horde), they don't need to stay there. You only need a token remnant pretty much like the void elves are on the alliance, to remain on the horde, and shift the full force and presentation of the Thalassians and night people to be present on the alliance which is where both blood elves and Nightborne come from.
That's why. The horde has to lose for the alliance to gain. This is the way.