Good question. But look at it like this. As I stated before in other versions of this question (Why didnt he bring Jack Harkness along) everyone the Doctor brought was someone who owed him a favor. (Save River) his previous companions don't owe him favors. It could be argued that River was already a part of this investigation and so that means everyone NEW was an enemy who owed him a favor. Alex Cross 16:22, June 16, 2011 (UTC)
Simple answer-becuase he didnt want to disturb their lives again. Sarah jane is too old to be fighting in a battle like that, martha is busy with unit, rose is in the parallel world and it would hurt the Doctor too much etc. Actually jack was to appear helping him but becuase john barrowman was filming torchwood series 4 he couldn't. Which is a shame cause I would like to see his reaction to the 11th doctor, both to the fact he is a lot younger than the 10th and that he changed again. I would also like to see how this doctor would deal with him and his "flirting". He probably would have been too busy flirting with amy, rory and everyone else to do anything! :)
In "Let's Kill Hitler", the TARDIS projects images of Rose, Martha and Donna. The Doctor tells the TARDIS to swap them straight away until it shows him someone's life he hadn't ruined yet (Amelia Pond). Maybe he didn't recruit former companions because he doesn't want to disturb them or he feels guilty because he messed up their lives. Rose is left stuck in a parallel Earth, Donna lost her memories and Martha's family were made playthings of the Master for a year. Or maybe he didn't want to bring them into a dangerous situation where they might have died. Jack can be excused because he was immortal and Rory would do anything to rescue his wife. His former companions also don't owe him any debts like Strax or Vashtra do.
The reason Sarah didn't appear is because she has her own alien problems on Earth to deal with. Out of universe, remember that Elisabeth Sladen would sadly pass away from cancer in April 2011 so she may have been far too sick to film for series six. As said earlier, Jack was meant to appear in "A Good Man Goes to War" but John Barrowman was busy filming Torchwood: Miracle Day!