Haven't you realized by now that Nintendo games are laced with cocaine to make you addicted?

Consistent quality

The character functions as a guide to the customer as to the nature and quality of the experience.

Other companies do so many low quality spin-offs with their mascot characters that it's no longer guaranteed you'll get what you're looking for if you pick it up. Nintendo never makes Mario and Pokemon games anything less than "very good".

This philosophy works for other developers - Valve, Blizzard, Grand Theft Auto and Call of Duty.

Nintendo isn't the only competent developer, they are the only ones who have remained competent. Spyro went skylanders, crash went away from naughty dog, pacman was led into different hands and so on.

Maybe because Mario games are bundled a lot and Nintendo releases dozens of Mario games every year.

Because they kept making good games? Sonic should try it sometime.

^This

Cause other companies get creatively bankrupt with their characters usually by the 3rd iteration

Also, Nintendo knows how to make games for the next generation instead of failing in the transition

Maybe.

Mario has remained popular, because his games are always at least decent! Spyro suffered from the sub-par PS2 games and Skylanders, Crash suffered from the PS2 games  and Sonic suffered from crappy games like Sonic 3D Blast and Sonic 06!