why does the author withhold the narrators name until page 12?

Typically the naming of a narrator such as Jim in this novel helps to build suspense and allows the reader to "get to know" the narrator and gain a picture of him BEFORE putting a name to the face the reader may have created. Since this is a first-person narration, it makes perfect sense that the author might have a picture he wanted us to see before we could add a name.