It's because the most likeable platforming characters come in pairs.
Banjo and Kazooie.
Sonic and Tails.
Ratchet and Clank.
Jak and Daxter.
Kirby and a helper/Gooie.
Spyro and Sparx.
Crash and Aku Aku.
I could go on but I think you get the point. Maybe if the future Mario title was Super Mario Bros. _______ and had Luigi playable from the start, the series as a whole might not be deemed overrated as much as it is. Just my thoughts.
Discuss?
Pixel Art can be fun.