The head of a yeshiva is, actually, more than the spiritual leader of the school.
His influence often extends beyond the school and scholarly matters, to the field of ideas and ideologies.
Yeshiva students usually choose to study at a particular school out of deference to the authority and leadership of the head of the yeshiva.
Usually this emotional and spiritual bond is maintained in all its force even after a student leaves the yeshiva, often throughout one's life.
Clearly then, the influence of the yeshiva heads is liable to be much greater than that of the rabbis of the community, who are themselves former yeshiva students with their own proclivities to such an influence.
Furthermore, the existence of so many yeshivot creates a whole class of laymen who are often authorized as rabbis and who have an education equal to that of a rabbi.
Such a congregation would require relatively little of the routine halachic-legal services of the local rabbi, and, in fact, he is frequently in the position of an ideological or political opposition.
What is more, a yeshiva scholar who wishes to devote his life to religious action will generally prefer to take up a career as a rosh yeshiva or as a teacher, or else as an official dayan (rabbinical judge).
Far too often, therefore, those who become community rabbis are frustrated individuals for whom it is second or third choice of career and who, even in the carrying out of their task, will dream of establishing a yeshiva as a fuller expression of their personalities.
–Rabbi Adin Steinsaltz
From "The Rabbinate in Israel," in The Strife of the Spirit by Rabbi Adin Steinsaltz