Thomas J. Reese from America Magazine, a Jesuit-founded journal of Catholic opinion, in his article on Papal Transition:

Most modern popes have felt that resignation is unacceptable. As Paul VI said, paternity cannot be resigned. In addition, Paul feared setting a precedent that would encourage factions in the church to pressure future popes to resign for reasons other than health. Nevertheless, the code of canon law in 1917 provided for the resignation of a pope as do the regulations established by Paul VI in 1975 and John Paul II in 1996. However, a resignation induced through fear or fraud would be invalid. In addition, canonists argue that a person resigning from an office must be of sound mind (canon 187).

I never thought of this whole business of papal resignation from the angle of political pressure before. Faithful popes could be pushed out by unfaithful political or clerical forces in order to institute those more malleable or agreeable. I know this would not lead to heretical popes, but it could lead to a serious weakening of the papal office if this set a precedent and that precendent was abused.