The man put Dr. Mario in the game simply because he wanted to justify putting in the music
There is no great tome full of secret Sakuraisms that can decide who gets in and who doesn't, the only criteria he's flat out said is that in order to be in the game the character must:
1) be a video game character
2) have been in a Nintendo game
There are actually four criteria that Sakurai actually has established. Taken from the Mach Rider thread (Also in the Mega Man thread, and various threads everywhere) :
1. The character must make someone want to play the game.
2. The character must be unique.
3. The character must fit into the game.
4. The character must contribue to the game balance.
And we all know Sakurai is a wierd guy, but it helps to look at his past decisions to understand his current motives as best as you possibly can. It's just the way people are.
As for Geno, I
love Geno, but I honestly think people are deluding themselves. Is he popular? Yes. Is he amazing? Yes. Is he as amazing as Mega Man? Debatable. Is he as popular as Mega Man? No way. Considering Sonic's placement was a popularity contest, unless Geno wins the "obscure" third party choice, if there even is one, he isn't going to win the popularity contest.
And I
hate to be that guy. I really do.
It's great to see such an influx of new people though. Welcome to the boards, those who joined in the past two weeks!