Just too many episodes. When I watched the first 3-4 seasons in one go... I used to get nightmares with daleks and cybermen in them. Not fun. Now, there is such a long wait... I mean I heard there would be a eight month wait after the Christmas special. That is insane. I would love to go back in time and watch it again as a newbie. When I started watching it, I had no idea it was so huge. I just thought it was some cult show that ended. Realized later on that it is something that would probably last another 50 years 😀