What about Spyro? What about all the other maskot games? If you're asking why Spyro isn't as popular as he was a decade ago, it's because there hasn't been a proper Spyro game for quite some time. Nintendo, on the other hand, makes several Mario games a year and has been doing so for the past 3 decades. This increases mindshare and all that stuff. See Call of Duty, Battlefield, FIFA, Madden, etc. Same with Pokemon-- it too is an annual franchise. Nintendo games aren't special. If you make a game every year (or in Mario's case a ton of games every year) you're audience will grow.

Nintendo just makes a ton of games for their franchises over a small period of time. See what happens when they release the next StarFox or Icelimbers or whatever franchise doesn't get a annual/semiannual release.

Look at this list of Mario games releases:http://en.wikipedia.org/wiki/List_of_video_games_featuring_Mario

The simplest answer?

Because Mario games are still, and have always been, consistently GOOD. You cannot say that about all Sonic, or Pac-Man, or Jak, or Crash, etc. etc. games.

Also, it just so happens that Mario rose above just being a popular game character, to being a pop icon. He's burned into the public consciousness in a way that other cartoon game characters simply are not. Sonic came close, a lot of people who have never touched a video game are aware of a "Blue Animal Guy" named Sonic. But EVERYONE knows who Super Mario is. In the 90s, it was stated that Mario was more recognizable worldwide at that point than Mickey Mouse, and that's saying something, because Mickey Mouse, despite not really (sadly) being used by Disney anymore, is STILL one of the most recognized pop icons of all time.

For Mario to even be in the conversation with Mickey Mouse, really says something. For as many other great "cartoon" game characters, or game characters period, as there has ever been, none of them really come close to Mario in terms of appeal and popularity, with the young and the old.

because only geniuses work on mario

Ouch, that kinda made want to give Sonic a hug.

Mario is happy. He knows when it is time to get to work, and he knows when it is time to relax. He isn't very perceptive, but he really loves his brother, even if there is some competition in there. 

Think Disney and you will understand. They reach gamers at impressionable ages and those gamers have their parents wallets as well. If they have children and their parents sales are sure to be high. They keep their universe tight and uniform like Disney with recognizable faces.

Its basically the only series/theme that sells nintendo consoles besides one or two other nintendo IPs. Its why I bought mine and about all I own besides sonic for wiiu. just like the wii the gen before.

How do you know he is happy?

Are you sure he loves his brother?

How do you know that Mario isn't just a pawn in Princess' Peach continued world domination.

HOW MUCH DO YOU REALLY KNOW?!

His happiness is shown in multiple cases. 

Play Luigi's Mansion Dark Moon

As for everything else I said, his lack of perception and his love for relaxation came from Mario Sunshine. 

Happiness is a emotion not a personality. Mario has also been shown to be sad on many occassions. Luckily he has different feelings or he'd be a robot, if not a 2D character. Lack of perception is a synonym for confusion which is literally a status condition in pokemon akin to paralyzation and sleep. As for love of relaxation, that tends to be the case with people on vacation, unless they are workaholics.

The truth is Mario is an Enigma.