The McCanns and their friends were interested only in finding Madeleine, or so they said. They thought the reconstruction requested by the PJ wouldn't help to find Madeleine so they refused to take part.
On the other hand the couple were arguidos, meaning that the PJ suspected them of being involved in their daughter's disappearance.
Taking part in the reconstruction could have shown that the group's timeline and their accounts of the evening were correct. Why would they think that wouldn't help?
Especially when they said they would do anything to help - especially when the said facts remain unclarified.
IMO it obviously had something to do with the timeline - it has never been checked.
You would have thought also the mccs themselves would have wanted to know where everyone was and what they were doing.
IIRC the tapas were not all known to the mccs - so if the mcs thought an abduction took place you would have thought that everything should have been checked out and not by assuming it was by a stranger IMO.