There are plenty of engineer trainers of other races in the game. Gnome and Goblin are Specializations of Engineering.
You could say that it wouldn't make sense that Tinker class gets added without Gnome and Goblin options, and use that to make your case. That would work since Tinker is associated with Gnomes and Goblins.
You can't use it as a means to exclude other races that have shown an aptitude in engineering, especially when your reasoning is opinion-driven (but its too magical, but they don't have a Tech racial leader, but they don't have racial Engineering specializations!)
None of your reasoning would apply to a new class that has no racial exclusivity. Death Knights can be Gnomes, Draenei, Worgen, Goblins and now Pandaren. These other races were not even in Northrend at the time of the creation of the Death Knights. By your logic, the Death Knight should have been exclusively Human and Undead (and to an extent, Orc) because that is what all Death Knights NPC racial leaders were and those are the only races at the time shown to use Necromancy.