I can't think of one Sport, that I didn't at least made One wager on.....
In my Lifetime I have Bet on MLB, NFL, NCAA, NBA, NHL, Tennis, Golf, Horses, Soccer, Boxing, MMA, Olympics, etc....
Guessing you overlooked autoracing. Anyone could come up with more and more obscure sports. But going by what is listed in the dropdown menus at ESPN.com and sportsillustrated.com, that would leave lacrosse, cricket, rugby, cycling. Also "action sports" (x-games?), track-and-field and figure skating as Olympics is vague
For myself, there are a good number