It's probably as Canary42 says but, even if they don't film in batches, knowing that the coaches will be wearing the same outfits each week will make it a lot easier for an editor if there's found to be a need to add in an extra shot after the filming of an episode has finished and the team have all gone home.
For instance, an editor might decide that he needs a few more 'noddies', or similar reaction shots, from one of the coaches. If that coach wears the same outfit for the next recording, it will be really easy for those additional shots to be filmed at that time.