The rumor is (for what it’s worth) that Benedict was too emotionally devastated by the scandal surrounding his (well really precipitated by) his personal butler and the Vatican Bank to continue his pontificate.
Benedict, as he aged, did not have a Ratzinger as his strong right arm, as John Paul II did , especially during his last five years. But John Paul took over a Church that was rent by faction and as Ratzinger played a an every growing role , the liberals saw themselves losing the battle, and they resisted and waited their time. Liberals see the Church as a vast NGO with clerical trappings, a giant-sized version of the Episcopal Church.