both goblins and kobolds are horribly underappreciated because it's easier for devs to add things for the filthy casuals to kill rather than talk to. And if they made them into playable races, then they would also have to make a new skeleton rig and brand new animations, with uniquely modeled armor sets and clothes.
Why spend any time in cool things like that when you can just add a fully or semi-naked woman into the game and watch as every moron either buys the game or talks about it online.