the reason why, is because Black kinda lets him be since he doubts that brud will do much for defeating him, so basically he focuses on killing the others since he would be aware that they could stop him if they had good enough plans, maybe? and somehow brud is aware of this.
this is also an alternative universe aswell. (sorta)
sorry if it's a bad explanation, or if it's not enough description for this to be your answer :(