There are a lot of players around the world. Not every games are favorite in all region. Some likes soccer, some maybe esports or some cricket or others.
Which one are you mostly placing your bets on? Besides, do you think gambler don't care which sport it is. I have seen gambler who don't have any fucking idea about cricket but he used to bet on cricket.
If a gambler bets on the game that he did not know, chances are he is only testing his luck and if he is lucky he can still score a win. Second reason that I can think of is there are promotion on that specific game, that is why they bet on it only to take advantage of the on going promotion. In sports betting it is now easy to copy bet from the other player, if you know the bettor, you will trust his bets.
In my case, I am not usually into sports but I do like esports, dota games to be specific. I can bet on the game any time of the day. It is important to bet on a game that you like because not only it can increase your chance to win but you can also enjoy watching the game.