Because C-17 is one of a number of transport aircraft types flying from A to B. The P-8 will be required to be on station over an area of interest (whether over land or sea). Assuming that there are on average 3 aircraft available for tasking, one could not keep 24 hour coverage in one area let alone two or three. Additionally looking over a history of operational Nimrod deployments, there were frequently 5 or 6 airframes deployed. The P-8 may well be more capable than Nimrod MR2, but it still cant be in two (or more) places at one time.