Because bringing back Brutes would ruin the story.
1.The Covenant lost the war in Halo 3, and the Elites (that are on the side of the Humans wich are the majority by far) are mopping up the remaining brutes.
2. Even if there were enough brutes left to fight, they would never ally with Jul Madama, Jul Madama is an Elite and ALL Elites hate brutes.
3. If we have to fight brutes again like in Halo 3, then that would defeat the purpose of Halo 3, Halo 3 was about finishing the fight, ending the war. Bringing back the Covenant in Halo 4 messed up the story, although technically they arn't actually Covenant. They arn't even an army. They are rogue pirates of traitorous Elites who hired grunts, jackals, and hunters. And they will not hire brutes, as Elites hate brutes.
