Well, I remember in Hush (and I think Hush was partially responsible for the renewed interest in Jason Todd) Bruce flat out admitted that Jason didn't have the skills to be Robin, and I think it has also been implied elsewhere. It's not hard to imagine that Jason would get the impression that a lot of people were convinced he just wasn't up to task (it certainly wasn't uncommon to see that attitude from the fans) and someone like Tim was much more suited to the role.