If Doctor Strange has done an equally good job creating its lead character, then it can focus a sequel on other things.
This would mean introducing new villains and potentially also other aspects of the world that could draw us deeper into Doctor Strange.
The idea here, it would seem, is that The Dark Knight was able to do some interesting things with its characters, specifically, its villains, because the introduction of Batman had been handled well in the original film, so the sequel didn’t need to spend any additional time creating its lead character.