The Horde is, as of right now, almost entirely blood elf.
If you added every non-blood elf Horde race together, you still would not out-populate blood elves.
It is because of this race, and
only because of this race,
the Horde currently has 1% more players than the Alliance. This was Blizzard's solution to the old faction disparity, which was so laughably bad that
Alliance used to out-number Horde about five to one.
And now Blizzard decided to actually give the Alliance high elves. Slightly reskinned blood elves, in other words. At last the "pretty race" everyone wants to be is on the "good guy" faction everyone wants to play, along with reskinned draenei (another very popular race.) And in return? The Horde receive skins of tauren and trolls,
which basically nobody plays whatsoever. Reskinned night elves too... formerly desired, now nowhere near the top of most-played.
This is Blizzard's very official way of killing off the faction. Horde been on an upward trend since the start of Legion thanks to demon hunters, but now it's going to hit absolutely rock bottom in terms of player count. You can see this right now on any private server of your choice: wherever you have the option to pick high elf or blood elf, the high elves outnumber the blood elves immensely. You can even see this sentiment twinkling in some of the salivating detritus inhabiting these forums.
The theme of the expansion is
quite literally faction versus faction warfare, and Blizzard has just implemented so many stops and favors for the Alliance they may as well offer sales on faction transfers too.