Why did Sai Baba break his promise to Bapusaheb Dev and not show up for the Udyapan feast?

This question is based on a common misunderstanding of the events. Sai Baba did not break his promise to attend the feast. As Chapter 40 clarifies, he attended the ceremony in the guise of a Sanyasi, accompanied by two other people, exactly as he said he would. The issue was not Baba's absence but Bapusaheb Dev's inability to recognize him due to a 'veil of Maya' and his own delusion. When Dev later learned the truth from Jog, he was filled with shame and remorse for having doubted Baba and failing to perceive his presence.