Red Or Black is prime time because it rakes in the ratings. If it didn't, they wouldn't have it on. It appeals to the same people that watch shows like the X-Factor because it is unintelligent, easy watching, time-filling garbage that takes no effort to watch. It's the fruit machines of TV. Literally. Those banners are extremely colourful for a reason, as are the lighting and studio sets, the dramatic audio. Everyone knows it is rubbish, but look.. it's so shiny.
Ant and Dec have discovered the lowest common denominator for television shows and are reaping in the cash. They aren't even milking it dry yet - there is worse to come I'm sure.
This is not a reflection on the type of person that watches it. They are not necessarily unintelligent, but many people just like to switch their brain off and simply observe "stuff" on the screen in front of them.