Currently in the game I'm running, one of my players is constantly excited about the game. Outside of the game, he writes pages and pages of material about his character, and consistently adds new facets to his backstory. This is great for me to work with, since he's creating a treasure trove of plot hooks and character points. The issue comes with the other players. Because I'm receiving so much rich lore from this character, I tend to have less for the other characters to explore. It would be unfair to tell this player to stop creating what he loves doing as part of his game, but I want to find ways of including the other characters' backstories into the game equally.
What can I do to avoid centering the plot around this character, and incorporate the other player characters more deeply into the story? It's a Pathfinder game, if that helps.
EDIT:. Accepting this is less acceptable of an outcome here, as my my players have at one point explicitly or implicitly expressed their desire to be more included in the overarching plot.