That was about two years previously (well, two years, or is weeks depending on comic-flexi-time.)
I'd say the loss of his father was too fresh, his (irrartional, but that's never stopped the Bat-clan) guilt too deep to take on a father figure who genuinely would have TRIED to be another father to him. It would have felt like a betrayal.
After the "One Year Later", he'd had time to put some distance between himself and the loss of his father. He still cared passionately about him, but he was gone, and Tim now knew that, and was ready to move on.