Because he doesn't really do anything. Within 5 minutes I know what Vader is all about and his goals and motivations.I always think it's funny how much backlash there is against Boba Fett because 'he just looks cool'. As if Star Wars isn't littered with shallow, vaguely defined characters who succeed primarily by virtue of cool visuals or simple attitude (Darth Maul, R2D2, Chewbacca, TR8R, Phasma, Jabba, BB8). Even Vader himself was more visual presence than anything else back in the first movie.
Boba Fett is a bounty hunter... and not that interesting.