R.O.B isn't actually a Main for me, and not even really a Secondary since I haven't truly mastered him yet.
But honestly, its how he plays and the functions of the Gyro and how... fluent he feels when I play him. He also happens to stuff out characters that beat out how I play Little Mac quite a lot of the time, specifically other Little Macs, Mario, and for some reason Fox, all three of which (minus Fox to a limited extent) I partially struggle with as my good 'ol main. Landing with Lasers and Robo Hoo-Hah is always a pleasure, and the ability on the one Animal Crossing with the side-to-side moving platforms to up throw suicide... Ah, glorious.
Also, ARM REFLECTOR IS STUPIDLY FUN TO USE.