Nobody wants to lose their money easily and gamblers are not exempted from this fact, that's why people will want to bet on specific sports they have good knowledge of so they can have a good chance of winning in their different trials. I'll be very confident in placing an accumulator bet on a sport am knowledgeable of for it's that confident that creates the believe that I can have a chance to win compared to when I would be betting on sports I am a novice to.
It is very common. In gambling luck is an important factor. But I will try my best so that I can win there and that's why I will try to bet in certain games. I will bet on the game about which I have good knowledge and on which I have more chances of winning. It's not just me who do sports betting, many of them bet on their favorite team's or the teams they know everything. Moreover, a gambler does not master all games. So everyone gets into the habit of betting on specific games.