Since Sir Lore of Admirals isn't here I will try my best to help you if he doesn't mind. My personal opinion is that 343 wanted us to battle a familiar enemy, along with the ability to add whatever they want to them. A forerunner race can have unlimited potential but the covenant is just a friendly face we are used to for the last 6 Halo games. It wouldn't really feel like Halo if we weren't fighting the alien races we know and love. So I think 343 wanted to still keep that Halo theme.