Most teachers in my school put out a sign up sheet at open house and BTSN. While I usually have plenty of volunteers, the have been years when I haven't had a room parent. There have also been years where I've had co-room mom and one where several moms did rock-paper-scissor to decide. One year, a mom announced to me that she and two other moms would be my room moms, and they would coordinate all aspects of our volunteer list.--It was great! I think some teachers want greater control (perhaps due to a negative experience), in which case they prefer to ask someone they know has done it before, etc.