Because an enemy of the family could approach him again in the future, just as Roth did. Because he was so jealous of Michael, he might betray his brother again, especially if someone promised Fredo that it would be something good for him and good for the Family.
I keep asking, how? Michael exiled him from the Family. He was only allowed cursory contact with his mother. He was banned from the Corleone hotels. It took Connie's entreaty to get Michael to give Fredo even a brief respite bewteen their mother's death and Fredo's murder. So, what is it that someone expected to obtain from Fredo in exchange for a promise of something good?
