Daystar Clarion said:
Much like Bobba Fett, Darth Maul was just a cool looking character that a lot of people thought was interesting.
They aren't really characters, but they're certainly iconic to the franchise.
Boba Fett was a better character than Maul was. They actual did develop him more despite his lack of screen time and lines. They used his few actions, lines and the reactions of others to emphasize who he was. Boba Fett is one of the only characters in the entire series that was able to talk back to Vader and live. The only others (who weren't key members of the rebellion) were Grand Moff Tarkin and the Emperor. He was the only one to figure out where Han Solo went. He was the first to draw a weapon on Leia when she pulled the thermal detonator in Jabba's Palace. They made sure everyone knew he was a bounty hunter, he was smart, cunning, and exceedingly good at his job. He was also well respected in his field and generated intense fear. Remember how Han reacted just hearing he was around? Fortunately this was over 2 movies (3 if you count the special editions, which I don't). But even in Empire Strikes Back, you had an idea of who he was. He had some depth.
Compare that to Maul. He was Sidious' apprentice which made him a sith. That's it. Oh and he has a cool weapon. They tell you nothing else about him. He follows orders. Yay.