Because Sprint hasn't approached Apple about it yet. Sprint will claim that Apple does not allow it. But Apple will say that there are certain channels that Sprint must go through to have it put on the iphone and that Sprint simply has chosen not to do this. They both point the finger at each other.
It was internal communications handed down to Sprint employees. You can also go to www.sprint.com/tvguide for more information about what is included in the Sprint TV channel lineup.