Cause companies can never change their minds down the road, right? Especially not game companies, no no no.
If Microsoft made CoD Xbox exclusive, they could make a lot of money selling their consoles & cloud gaming service to the masses. Being that Microsoft is a corporation (i.e. their objective is to make money), do you have any reasons WHY they shouldn't make it exclusive? The only argument I can think of is that, when they crunch the numbers, they will make more money selling CoD on other platforms than would getting customers to buy Xbox's specifically for CoD. This argument will decide, in the future, whether CoD remains multiplatform or not, nothing else.