There never really was a persuasive argument (imo) against adding High Elves to the Alliance when you got into the details of the matter, especially the storyline since Wrath of the Lich King (not to mention TBC — I bet there’s an entire new generation of people who skipped Outland in retail WoW (or just played Horde and didn’t know) that are now learning that there really were Alliance High Elves even back in TBC, not to mention Classic — can’t wait for Wrath Classic to add on to that).
In the end, Blizzard split the difference by adding Void Elves and then expanding on their customizations to make them less Voidy.
Adding a new race, while RPG-thematic regarding character choices and ludo-narrative distance, does still require adding new racial abilities that require balancing and at least some lore and writing. In short, even though the art assets are mostly (if not entirely) there, there are still SOME costs associated with adding them in. Easier to just add customizations to Void Elves.
(Not saying I like that pragmatic look on it — I would like non-Voidy Silver Covenant High Elves — but that seems to be the case based on available information).