it isn't because the ending of halo 3 pretty much raped it up. I haven't played the campaign in a while but the covenant fell apart and UNSC won the war. The flood were destroyed 'apparently' so who's left to fight? I would hate a game all about fighting flood cause it would feel like a cheap rip off of the great halo trilodgy. The only thing we can do now is fill in the gaps between halo 1, 2 , and 3. Or remake some of the games if there desperate. It is time for a change and it is happening in the near future.