In short, because it would lower the standard for playable races in terms of distinctive identity, in particular between factions.
Blizzard has been committed to introducing new and creative races for either faction, and it's important that they maintain that pursuit, or we risk losing a wider breadth of options in favor of filling in the insignificant details in between. From Blizzard's perspective on this specific topic, the issue is that the fantasy of this particular race is embodied by Blood Elves, and they are something that is intended to make the Horde appealing. To recreate that fantasy on the other faction, especially with 2018 levels of quality, would take something unique and valuable to the Horde, strip it of its distinctiveness, and overshadow its quality. That would only serve to make the Blood Elves, the Horde, and in turn the game as a whole less appealing, all for the sake of adding redundant pieces that lack the unique identity we have come to expect for playable races in terms of visuals, fantasy, and story. That is not a direction I want to see Blizzard take the game, because I think it discourages creativity and panders to a minority at the expense of the game's overall quality. I believe that adding them would placate a small group of players in the short term, but wouldn't be worth it to any of us in the long term.