Or we can look at the evidence. Here's census data from active 120s broken down by Faction -
You can see that for Horde, Zandalari already have a good representation nearing that of Undead.
The other allied races (bar Highmountain) have about 4% representation, beating Pandas and bringing them level to Goblins. Not bad for some races that have barely been with us more than a year.
You can see from Alliance that Void Elf have been very popular, and they're more popular than Dwarf, Gnomes and Worgen already.
BUT, all of the other Allied races have half the amount of representation as the Horde ones do. They simply aren't as popular.
While Zanalari have been popular, Kul Tirans are easily the least played race in the entire game.
It's obvious from the data that there simply isn't the same interest in most of the Alliance allied races as there has been for the Horde.
Gnomes were already one of the least played original races in game.
There are currently more people playing Highmountain Tauren in game than there are playing gnomes. Making a gnome variant seems entirely redundant.