It really depends on Blizzard's business model regarding selling the Allied Race feature. What do they want with it? Do they want them to be hugely popular and bring massively players to play them (as much players as possible), or just a prestige/elite reward to players that commit several months of their gaiming time to unlock them?
As they are locking them under big time consuming requirements (see my previous post about them), i presume they prefer the second business model, aka a reward. As not everyone has patience to accomplish all requirements per race (each race unlock is different from the others, which requires different time consuming efforts for each race, and also for each faction), this clearly is a wall that slows down the creation of allied race characters.
Both in NA and EU realm pop statistics, we can see that their population aren't big. I agree that Mag'har Orcs and Dark Iron Dwarves are still recent, but the other four races have been playable since january/february 2018 for people who pre purchased BfA, so their numbers should be much higher than they are today.
If we sum the percentages of the 6 allied races in realm pop, either on NA or EU on the total available range levels (1 til 120), we can see that that total percentage is aproximately the same as the sum of both Horde and Alliance Pandaren (!!!), or the sum of the combined population of gnomes and goblins. Being one of the biggest sellouts of the expansion, isn't it rather odd that their total numbers haven't surpassed the previous races? Counting only level 120 characters makes the statistics different, but it isn't that much different as we would expect: We can see that nightborne and Void elf populations have surpassed Pandaren, Goblin and Gnome populations (in NA even Dwarf and Worgen populations), but the highest share of level 120 chars is 3,3% to void elves, which isn't much compared to other popular older races.
Blizzard could improve the popularity of the races by reducing the requirements i believe. But then my first question pops up again: what does Blizzard really want to do with the Allied Races feature?