In Backgammon, why is the match length almost always odd? And why are 9-point matches twelve times less popular than 7-point matches?
Largely it's by convention, but the historic cause is the way optimal doubling cube strategy varies with the match length. In the most easy case, in a 2-point match it can barely be wrong to double as soon as you are ahead, and far too easy to leave doubling too late - which means the match becomes a single game the vast majority of the time. In which case you might as well play a 1-pointer. The other small even match lengths suffer similar oddities, but less pronounced as the length increases. Odd match lengths tend to have more "normal" doubling strategy in the early stages. 

On at least one server I play on, 2-pointers are frowned on because they are perceived as a way of playing a 1-pointer for double the ratings value...