The Office went through all of its kooky manager plotlines because Steve Carell's career trajectory outpaced the show's initial popularity. The second he got too big for the show, everyone knew it, and much ink was spilled in the writers' room trying to anticipate and/or compensate for his eventual departure.

Jim was given some upward mobility because he's the protagonist, and the audience was rooting for him (and Pam) in a personal way. Nine seasons of Jim's being stuck in a dead-end paper sales job, with no hope of escape, would have been extremely depressing. I realize that's how the UK version chose to play things, but it would have been too bleak by half for the US market.