Because that is what is usually asked for! But seriously, a principal looks at things from a managerial perspective, where as a teacher's perspective is that of day to day operations. So if a teacher speaks highly of you, that will not carry as much weight in the decision-making process as the same words spoken by a principal. If the teachers are your peers, they might let the fact that they like you and get along well with you color their assessment of your job performance. The principal is one step removed from all of that, and is considered to be more objective in his/her assessment of your skills for the position to which you are applying. Make sense?