...because the man likes to ad-lib. Apparently, this isn't something that the papal communication team likes too much. From totalcatholic.com:
What his audiences find amazing is how easily the 78-year-old pope can stand and deliver an impromptu talk or sermon that ranges over Scripture, patristic writings, social ethics and pastoral policies. "He's using a fluid form of speaking to deliver a content that is very pastoral. It's on a high level, but you can see that his audience follows it," said Joaquin Navarro-Valls, Vatican spokesman. When the pope presided over a Mass in the Sistine Chapel to baptize 10 infants in early January, he was supposed to deliver a sermon presumably prepared by his staff. The text, released to journalists ahead of time, was nothing special. Maybe that's why the pope pitched it. Instead, he stood beneath Michelangelo's fresco of the "Last Judgment," looked out at the small congregation of parents and relatives, and began, "Just what happens in baptism?" Then he extemporized on the topic for 16 minutes -- twice the length of his planned homily. The Vatican press office, meanwhile, sent out an urgent disclaimer telling reporters to ignore the prepared text. The pope did the same thing when he visited a Rome parish in late December, preferring to wing it through a sermon on the Annunciation and its significance in salvation history. He began by examining Scripture and the account of the angel's first words to Mary. The Gospel originally used the Greek word "Kaire," which contains an element of rejoicing that is missing from the "Hail, Mary" translations in other languages. He connected Mary's reaction with feelings of joy and fear people sometimes feel toward God. Nowadays, fear and apprehension seem to prevail, he said. "If we look around the modern world, where God is absent, we have to say that it is dominated by fear and uncertainty: Is it good to be a human being or not? Is it good to be alive?" he said. As one longtime Vatican observer commented, "Even in his spontaneous talks, the flow of argument and the citing of sources is impressive. It's as if he can reference 2,000 years of Christian thought in his head." At his weekly general audience, the pope now regularly punctuates his prepared remarks with explanatory asides. The off-the-cuff comments are typically more direct and succinct than the written reflections. At the Jan. 11 audience, for example, after mentioning the third-century theologian Origen -- probably just a name to most of the pilgrims in the audience hall -- the pope gave an impromptu lesson on his thought. Origen, he said, believed the fundamental difference between man and animals is that man is able to know his Creator. "It's important in our time that we don't forget God, amid all the other knowledge we've acquired," he added. That knowledge can be problematic, even dangerous, without an awareness of God, he said.