Short answer – roulette poses a greater risk to casinos as it has a lower house edge than slots, making bonuses a form of free money. As a bonus provides free money, the casino risks losing more money on games with a lower house edge, such as roulette, than games like slots with a higher house edge. Additionally, some players may attempt to exploit the bonus by exclusively playing games with a lower house edge to increase their chances of winning, which can lead to financial losses for the casino. Therefore, restricting roulette gameplay with a bonus serves as a way for the casino to minimize their financial risk.