Well, I imagine if they were a surprise that had to do with BfA's plot, being BfA-relevant races, they'd want to keep that a secret even at Blizzcon as it would be a nice surprise.
And yeah, the one set does irk me. Rajani Mogu and Waveblade Ankoan as the last two BfA races would've been nice. Part of me feels like the list as is should end with a BfA race, and then there should be a new one for any potential Shadowlands or (if Shadowlands indeed has 0 allied races) expansion beyond races. But that's just me. You could argue that the initial four races were Legion only and therefore already break that notion.