1)Who said they aren't canon? Who said there even was a canon to begin with? 2)As far as I'm concerned, every Mario game happened. 3)Why would you want to act like a game never existed?
1) Well, for one thing, I don't think Bowser would meet with his enemies just to have a race/play golf/play tennis/etc. If he did, then don't you think he would at least
try to kidnap the princess? Especially since she's right there, none the wiser?
2) Even the awful
Hotel Mario? Even Mario is Missing, Mario's Time Machine, Mario teaches typing and all the other Mario educational games?
3) In the case of games like Hotel Mario, it's because the game is bad and there was no influence by Nintendo in it's creation, was not on a Nintendo system or was not published by Nintendo.
In the case of the Mario spinoffs, it's because the game(s) have no real place in the Mario timeline and/or make a mess of the story (Such as the example I pointed out earlier).