Which WE ALL KNOW they don't (if the McCann wanted to keep the investigation running they could have asked so for 20 days after the publishing of the AG report) and since then at any time, as long as they provided just a shred of evidence that they would co-operate, just by saying so by e-mail or a registered letter.
But the OP is why did they leave the door opened. There is no proof they ever did leave it open but for their own testimony which arrived rather late, after all the other possibilities of entering the apartment by a stranger were proved impossible - no jemmied window and no forced front door.
Now, even if some (many) people accuse the McCann of bad parenting for leaving the children alone while going for dinner, how many of those people would believe they would leave a door to the apartment open. A door that leads directly to the main street by a small gate that doesn't lock? A gate that is not visible from the place where they were just like the balcony door also wasn't. Even if you believe they are highly negligent parents, can you believe they would leave their children in an unlocked apartment from where their kids could get out without anybody of the group noticing?!
Furthermore, Gerald McCann proved to be a scrooge when demanding a lowering on the price of the apartment because there was no listening service as he was lead to believe by his friend Payne, would such a person leave his belongings alongside his kids in an open apartment?! - Yes, I am speculating. But think about it.