Ok, so from what I understand... Catherine had prior engagements so she couldn't do New Moon and Chris will be busy with finalizing New Moon during the filming of Eclipse, hence David Slade. Hope this helped you a bit.

Well Catherine had prior engagements and couldn't direct New Moon. Then with the mass demand of Twilight Summit wanted to release the movies on top of each other and with Chris directing New Moon he would be editing the film when filming would begin with Eclipse and supposedly that will be the same situation with Breaking Dawn.

UHHH its because Kristen Stewart is a little shit and they all hated her so they left!

Why would you have the same director for every movie? You gotta switch them around, because some may do better jobs than others. (;