Bonuses are limited to slots in most cases simply because the slot machines give the casinos very good odds. There are casinos that will allow people to play the bonus in table games, but this might be a specific form of a table game. The bonuses might be for specific games because the company wants people to play that game. This might be something that people will get for a brand new game, or it might be something that the casino does when they want to feature a particular developer that they want to work with or promote.