Mostly because of what Ronster316 said. Mario had, has and will probably continue to have one of the best controlling schemes and playability of any 2D and 3D platforming game in existance.
As for the spin-offs, most thrive only in the licensed name of Mario and quietly go down in sales after a few iterations, yet some also create one of the best gameplays in it's genre, like Mario Kart and Mario Tennis (and for me, particularly, the forgotten Mario Golf).
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
