Continuing in the trend of fairy tales with evil stepmothers.

I think everyone's characterization at the very start of the series was a bit one-dimensional (remember Mickey too?). Jackie included, but I think she evolved as a character. There was the Father's Day episode, she was actually very funny in the Christmas episode, and she actually got to travel in the TARDIS (mistakenly, but still!) in Army of Ghosts/Doomsday.

Also I think that seeing the Ten and Jackie dynamic might also have informed them that Donna would do well as a companion too, perhaps.