That's because overseers are called to oversee the flock...And the first requirement is that the overseer have a wife and family; to show that he can run his household well before he is put in charge of overseeing the flock of God...
Lol. Being a good husband and father ISN'T the same at all as being a good priest, vicar or minister. Success at one isn't a guarantee for success at the other.
A pastoral flock is NOT comprised of a wife and children. The flock is comprised of children and adults who are looking for spiritual guidance, not parenting or being a husband.
VERY different skills sets.
But tradition trumps Scripture so they can ignore that.