Why did the Messenger (peace and blessings be upon him) not explicitly designate Abu Bakr as his successor?

The Shaykh : The explicit designation was not made so that the matter would combine both the explicit text and the ijtihad (independent legal reasoning) of the Companions. For if he had explicitly stated it, there would have been no place left for ijtihad. However, he left the matter as it was, by indication, so that his caliphate would be established by both the explicit text and scholarly consensus.