because goblins are used strictly for being a villainous mob, and any time a goblin (or similar grunt) is used as a main character, it's always that stupid "what if the story was told from the VILLAINS' perspective?" trope, which just devolves into a watered down Wreck-It Ralph clone.
It's a shame too. Goblins are awesome; they're always portrayed with amazing team coordination and pack tactics. I'd love to see something centered on a group of goblins.
Also maybe it would feature a shortstack cutie.