The reason behind this is found in CoD3. Infinity Ward is owned by Activision, who want their killer Call of Duty franchise to appear on Christmas shelves every year. Grant and his team were not willing to rush their next project to meet such a deadline so they came to a simple truce: Infinity Ward got two years to make CoD4, and stand-in developer Treyarch took care of CoD3 in the interim. While certainly not confirmed, it appears that such an approach will be taken again in 2008, leaving CoD6 as the next likely Infinity Ward project. Well, alongside the development of more maps (confirmed), weapons (confirmed), perks (confirmed), attachments (confirmed) and a co-op mode (rumoured) as DLC for CoD4.