He definitely did it because of ego. The majority of the people he dealt with were all dead by the time he wrote that book. It was his chance to get the last word, to portray himself as a hero as a man of honor. The most comical part about the book is how he says he NEVER dealt in drugs or allowed his family to. Notice how he NEVER mentions carmine galante in the book? He doesn't once mention how carmine drove for him, he doesn't mention his as a capo or anything. Carmine Galante was a major player in the bonanno family but if you went on the book alone you would never know who he is.
On the A&E biography of Bonanno, they show clips of his (and his son Bill's) interview with Mike Wallace. In one clip, Bonanno says "If Carmine Galante, without my knowledge, is dealing with narcotics, I never know, and this I don't have to swear, but I can swear on anything."
