This is the biggest point I feel works against Tinkers. Once you limit to Gnomes and Goblins, you kill the entire class concept. It makes the most sense from a lore perspective, but gameplay has proven that Gnomes and Goblins are unpopular despite any design and flavour they're given. Gnomes and Goblins aren't unpopular because of lack of uniqueness or awareness. Gnome and Goblin Hunters are given special access to Mech pets. This still hasn't pushed their numbers very high, and they're most likely being rolled by people who are already Gnome/Goblin players. This severely limits the Tinker's demographic.
The second biggest point is how people generally view these diminutive races. It's not that they're unknown or lack specialness. Hearthstone even had an expansion devoted to Gnomes and Goblins (and yet another one with this latest Gadgetzan theme). It works in Hearthstone though because it's silly and whimsical. The races (and by extension, the class as well) will lack seriousness that gives people incentive to drop their main/alts to roll this new class. It's just unfortunate that the race meta is all about the pretty races, to the point where Elves had to be added to the Horde.
If we get a Gnome/Goblin exclusive Tinker, it will have a tough time breaking the meta and becoming a class that stands toe to toe with the rest. Doesn't matter how fun the gameplay is, it will be wasted if people lack a serious investment in the class due to the limited race options.
http://www.warcraftrealms.com/census.php?class=Hunter
This is why a Demon Hunter works and a (gnome/goblin exclusive) Tinker would not. Even if Tinkers become the most popular class for their respective races, it would be tough to break 30k total players, compared to Demon Hunters which already enjoy 80k for
each race. Even Monks, as few as they are, have a total of 100k players; 35k Pandaren, 33k Humans and Blood Elves, and 32k for every other race. Imagine if Monks were Pandaren exclusive.