Because not everyone can be winners. It's why beggars can't choosers.
Just look at Bubsy, he is the definition of what's wrong with the others.

Nintendo finishes their games. 

Let's see. The problem with Spyro and Crash is Activision! It's that simple! They destroy everything they touch!   The more recent Sonic games like Colors and Generations have been good, but the shittier games like Sonic 06 and Secret Rings left such a bad flavour in everyone's mouth that many gamers simply gave up on the franchise!

Because of the high quality of its main games

Because Mario and Pokemon have been with the same developer for some 20 years now. Crash was on the top of the world with Naughty Dog. After them, it kept changing developer constantly, giving no consistence. In fact, the first Crash post-ND still sold pretty well - but consumers pretty much gave up after they saw it wasn't on par with the ND games. Spyro suffered from the exact same problem.

In Sonic's case, it was both Sega's colossal screw up with the Saturn, and their inability to transition Sonic into the 3rd dimension - not to mention an unnecessary focus on story.

Digimon had its biggest chance at its peak, with the first season, Digimon Adventure. But I think that Digimon World was not what people were expecting. If they released something more like Digimon World 3 or Digimon World Dawn/Dusk, perhaps things would be different? We'll never know

I remember watching a video on gaming mascots of the 80s and 90s. Mario was talked about in parts of the video. 

Interesting stuff.

They know how to handle Mario games, they don't churn out low quality crap, every Mario game Nintendo created is 100% guaranteed to be high quality, and they change the formula not too little that the games get stale and not too high that the classic Mario vibe is gone.
But who's (whose?) to say an incompetent CEO takes over one day and runs Mario to the ground?

Mario games consistently have tight controls, brilliant level design and endearing aesthetics.

I also think Nintendo managed Mario well when he could have fallen off. Back in the NES and SNES days Mario was like Mickey Mouse in terms of cultural relevance. Many companies would have run such a phenom into the ground, but Nintendo showed remarkable restraint.

Then Mario 64 landed and shook the whole industry. How many series can redefine an entire industry twice? Yet, again Nintendo held back and no 64-2 happened. The Gamecube saw one entry in Sunshine and really it was not until the NSMB sensation that character was threaten by main line over exposure.

Nintendo have managed mostly successfully by splitting the series with 2D and 3D entries to appease different groups. At the core though they are unparalleled as plat formers with the Galaxy games in particularly being true classics.

I think Mario will see highs and lows, but as long as Nintendo manage him I fully expect to see him in quality major releases for years to come.

Mario as a character has zero personality.
Go ahead, try to describe his character without referencing:
*his job, clothes, heritage/race
*the various settings of the games he's been in
*any of the other character's in the Mario Universe
*any other characters in general

Because Mario hasn't been thrown around many different studios like Crash and Spyro did?