The reason, I believe, was that the Don was looking to Johnny to help Nino. There's a bit of dialog in the Don's office at Connie's wedding that points in that direction:
The Don commends Johnny for taking care of his former wife and his kids, but upbraids him for not helping his friends. "You couldn't help him [Nino] a bit?" asks the Don. "He doesn't have enough talent, he's just not big time," says Johnny. "And you, Godson, now that your voice is weak, shall I get you a job on the truck with Nino?" the Don mocks. I think the Don expected Johnny to help Nino as a kind of quid pro quo for the help the Don gave Johnny.