Speaking to Electronic Gaming Monthly, Bungie's Frank O'Connor reportedly stated: We're not dumb. We know that people want it and we're trying to make it happen.
I think the biggest problem for us for online co-op is that we have a situation where you can be in a Warthog with five troops, almost a mile away from the other player. That's a significant challenge.
And there's lots of design things you could do to prevent that from happening, but they would make it not feel like Halo any more. If we can make it happen in a way that works well, we will – and if it works badly, we won't.”