The more reason to keep adding allied races, but it needs to be something popular. I'd guess from the top of my head Highmountain Tauren don't get as much popularity, because despite great racials, playing a cow was never super popular. That's why I expect Zandalari will be a much bigger hit. Especially with paladins, adding paladins to Tauren that was already one of the least popular races together with goblins and such, so probably didn't change a lot.
One reason why they might not add vulpera is they're looks wise something between a goblin and a panda, and none of these races are strikingly popular.
Most popular horde races were Belfs (already got Nightborne as allied race), Orcs (got Mag'har) and Forsaken (room to add something?)