Because the brutes weren't part of the covenant yet.
Except they were. Play REACH or read the fall of reach. Pretty sure they only made one appearance in the book (that I remember) but they pop up from the level exodus to the end of the game in Halo: REACH.
brutes were in first strike not fall of reach you idiot
My bad. In my memory, during the mission where Linda was severely injured (which I KNOW took place before the Pillar of Autumn jumped away from reach) they encountered brutes. Its been a while since i have read the books though, they all sort of blend together in my memory. there is still the little detail that they appear in Halo: reach, so my point is still valid. Brutes were on Reach.