Tim can't sub in for Damian because you can't tell Damian stories with Tim.
Tim was created to tell basic Robin stories. A replacement for the OG likeable sidekick who was there to just be the audience surrogate. A position that Dick's character have evolved beyond. The typical textbook Robin that Dick Grayson was when he was first introduced before he became a more nuanced and fleshed out character decades later.
Damian is different, while he has been used to tell typical Batman and Robin stories like in the Batman/TMnT crossover.
His origin, themes and character type means that a character like Tim can't be used to tell Damian's stories.
Morrison's B&R
Tomasi's B&R
Williamson's B&R
wouldn't work with Tim Drake because doesn't have the relationship or history Damian has with Bruce Wayne or Dick Grayson.
Basically what @Jon Clark said.