Pope provides insight into selecting St Francis of Assisi:

http://news.yahoo.com/francis-turns-charm-first-meeting-press-111911747.html

Francis then described how he was comforted by his friend, Brazilian Cardinal Claudio Hummes, as it appeared the voting was going his way and it seemed "a bit dangerous" that he would reach the two-thirds necessary to be elected.
When the threshold was reached, applause erupted in the frescoed Sistine Chapel.
"He (Hummes) hugged me. He kissed me. He said don't forget about the poor," Francis recalled. "And that's how in my heart came the name Francis of Assisi," who devoted his life to the poor, missionary outreach and caring for God's creation.