People love Maul and Fett for the same reason that Alien didn’t show the xenomorph until like the second half of the movie: people’s imaginations will come up with something better than you can make almost 100% of the time. They pop on screen just long enough to look cool and maybe say one or two lines of dialogue to get the fans going, and then disappear. They’re the character equivalents of those lore-bait games that don’t have an actual story, just hints of stuff for fans to make YouTube videos about.