By the way if we look at this from a chronological standpoint, the BR's return isn't a surprise. The story states that the Battle Rifle is the successor of the DMR and though DMR was still in use after the BR was made by the UNSC, it was only used in certain military operations only. Otherwise the BR was the most commonly issued standard Rifle.
....and since the story of Halo 4 takes place immediately after Halo 3, there's no reason the BR shouldn't be there. I don't know about you but i think the DMR is still a harder weapon to use than the Battle Rifle no?