He came out with a prepared statement that he read-— as he is not able to speak off the cuff. Since this was an unplanned statement he HAD to make because of the news, there was no time to prep for prepared answers.
( Trump, Cruz, or Carson would not need to read from a piece of paper.)
If you don't mind my addition:
Since this was an unplanned statement he HAD to make because of the news, there was no time to prep for prepared answers to prepared questions that would have been previously distributed to the sycophantic WH press.