Except then you disrupt every single show airing on the block and risk some viewer attrition from those who don't follow schedule changes that closely.
Look, at the end of the day, if there weren't any other big pick-ups worthy of that slot, why not rerun Jack there? It's summer, when TV ratings are traditionally down regardless of what you do. Honestly I have to question the correlation vs. causation of any sort of block-leading reruns actively hurting ratings across the whole block. If people were already watching the other shows, they're going to continue watching them regardless of what airs at 11. Honestly I think it's more likely that what we're seeing now is a regression to the mean from the surge of new viewers from the super-promoted Jack premieres. You lose those excess eyeballs, and viewership drops no matter what's airing.