I think the reason is that Sol knew Tom favored the deal even before the shooting, and said he'd "try" to persuade Sonny and the capos after Sol kidnapped him and let him go. But obviously Tom was not successful in getting Sonny to come around. Why try Tom a second time? Perhaps Sol thought that Michael, as a "real" son to Vito and a "real" brother to Sonny, could be more persuasive.

"Personal, not business."