About a year ago, this question was posed to our bishop when he visited our parish. He replied that one reason why bishops aren't married anymore has to do with the iconoclastic controversy. If an iconoclasitic leader wanted to impose his will on the people, he went after the bishops, because whatever the bishop says, the people are going to be influenced to listen to him. For example, the iconoclasts would tell the married bishop, "We've captured your wife and kids. Unless you preach to your people that icons are bad, we're going to kill your family." So because of this, bishops were chosen from the celibate ranks, to prevent them from being influenced to preach heresy when their families were threatened. A celibate bishop seems to have less to be threatened with than one who has a family.
Granted, this answer was posed to a group of children, so it may be a bit simplistic in its rendering, and of course, there were other factors, such as the one about preventing bishops to passing on property/titles to their sons.