"I'm trying to think of some good excuse for the pilot or flight attendants not to tell the passengers but I can't think of one."
You are not a pilot from that statement.
There would be no reason to tell the passangers since every plane in the system at the moment of outage has clearence to a destination and no more traffic is allowed into the airspace.
Even if sll communicstion is lost until every plane in on the ground there is still no danger.
Even the airfield is clear for each landing in the specified time slot.
Sorry but that does not explain to me why passengers have to be kept in the dark, perhaps some in a state of fear or panic. If the plane is supposed to be going to destination A, and there is an announcement that we instead going to destination B, with no further explanation, that does not seem right to me.