They get removed because other programs take priority. Then when you use another program frequently that takes over. If you want to have programs you want to go & stay there is by right clicking the start menu/start menu & unticking the two bottom boxes. Then you pin the programs you want to go there. By doing this other programs will not be added to the list. (talking about pinned programs)
Can't be more precise, I'm not in windows. Will try to find some screen shots if needed.
EDIT: over here Jump List - Only Certain Programs?