Pretty much what vonGenf says. If you are a Priest in BattleMaster, you are one whose primary concern is preaching.
There is absolutely nothing that says that a Warrior (or any other class) cannot be deeply religious and have a strong desire to see his religion spread among the nobility.
However, for game balance, no one character can have access to all the different class functions at once. You might as well ask why Warriors can't Survey Administration, or why Traders can't murder militia.
