Why does Gandalf reappear at this point in the story? Why not sooner?

The story as it's told prior to Gandalf's reappearance is one of growth and change. Gandalf's presence is calming and powerful all at the same time, but at this point in the story, his appearance is also an affirmation of Bilbo's actions..... he's there at a point where many see Bilbo as a traitor. He commends Bilbo and provides him with confidence in a time of confusion.