Batman, Spiderman, Superman, Iron man get pins because they are major Hollywood action movie franchises. Comics have nothing to do with it. Comics may have originally created the characters, setting and tone. But mostly they have all moved long past being comic books. They became major network TV shows and Hollywood sequel franchises (rebooted, again and again). Some of those titles became major media standard fodder, generations ago. Some are more recent entries, like Deadpool. But it's all about the major Hollywood mega blockbuster. Obviously Spawn and Hellboy are likely some time or another, pending Hollywood investing heavily into those titles (again).
It's the same thing for LOTR and Hobbit, for another comparison.
Game of thrones isn't a pin because it originated from a popular series of novels. Why isn't there a Catch-22 pin, or a Slaughterhouse 5 pinball machine? Those are no-brainers, right?