5. Jack Has Outlived His Usefulness
The introduction of Jack in The Empty Child was a totally inspired idea. Dashing, handsome, heroic and pretty much ready for anything. He manages to save Rose from her own stupidity (but only because he thinks she has an 'excellent bottom') and then ingratiates himself with The Doctor for nefarious purposes. He is after anything and everything he can get until he discovers what his ploy has achieved in The Doctor Dances. He faces The Doctor's wrath and anger concerning the nanogenes which changed Jamie into what he became upon his tragic death. Jack is killed by a Dalek in The Parting Of The Ways, and then brought back to life by Rose. He's not immortal, but he doesn't stay dead for long. All well and good if you want near-immortality, but I suspect that Jack would have been quite happy to die and stay dead. Aside from a few more episodes, Jack's story is done. There is honestly no reason for him to appear again, aside from the fact that he's a lot of fun to have around - unless you're Mickey, of course.