Because after they claiming they would leave no stone unturned, after the dogs' intelligence, the refusal to answer questions and the lack of will to reconstitute the event, and moreover after having become celebrities, they just couldn't leave the stage without thickening the doubt about them.
They have never refused to take part in a reconstruction. Kate refused to answer the 48 questions (many of which had already been answered before) on advice from her Lawyer. As Kate knew nothing about Portuguese law and her Portuguese lawyer did - are you suggesting she was wrong to take that advice?
As for calling them 'celebrities '- that's just nonsense. The only time they appear in public is in connection with Madeleine. They may have become well known but I would no more describe them as 'celebrities' - than I would describe Ben Needham or Sarah Payne's parents as such.
I refer to my original point that if the McCanns were truly as disinterested in their child's abduction as Luz claims, then their actions over the last 6 years on Madeleine's behalf would simply not have happened.