Since Dr. Strange uses the power to turn back time not much later than the ancient one's death, why didn't he use it earlier to bring the ancient one back to life?
In my opinion, maybe the thought even crossed his mind, but he chose not to do it. Keep in mind that, as Mordo said, messing with the time-space continuum is dangerous, it can have collateral effects such as creating branches and time loops. Therefore, it should only be used in desperate times, like when the last sanctum has fallen and earth is about to be destroyed. Using the eye of Agamotto to bring someone back to life is not something admirable (kind of selfish to revive someone whilst endangering many more). I believe the Ancient One would not have approved of it, since she said: death is what gives life a meaning. To know that our days are counted and each should be enjoyed as the last.