It is kind of crazy that we got an offshot of original mechagnomes that look like cyborgs and not real machines before one of the oldest and longest standing allies of the original Horde as playable. Shame on you Blizzard.
There was only one gnome, and it was not the choosen one…
Jokes aside, darkspear and zandalar makes two.
The gnomes was only one. So this is probably the reason.
(And if you want the lore answer - the gnomes is highly technical - from the beginning, and a merge between the two was inevitable, thus gnome become cyborg gnomes. Now they are two aswell).
Maybe the next race will be troll of some sort. (or undead, because they are only one…sylvanas… just saying)