Probably a practical and financial reason such as the Catholic Church not allowing priests to marry and have children as a form of preventing priests passing on accumulated wealth or power to their offspring.
they're not now. but christianity is an old religion.
after it became the official religion of Rome their aristocracy increasingly took up bishoprics, wich were much more powerful then, and acted as great lords do. paul johnson's a history of christianity is a good read if you want to know (lots and lots) more
