I can guess too:

-Damage happened since the last bottom job.
-Damage happened since the last owners visit.
-Marine growth on the keel scars.
-Amount of damage done since the water intrusion started creating more damage than just the keel box fractures and stress cracks.
-In many cases you can see the stress cracks behind the keel by just lifting the floorboards without putting someone in the water.

What I am curios about is how this damage went hidden and why there was no underwriter with a charter operator involved to handle the loss.

To the charter boat keels. Just put a mask and fins on next time you are in a crowded anchorage. The battle scars going way up the keels are more than I could ever count.

Last guess... Did they take the boat from the operator, settling, and ending that contract before the damage was discovered?

Would a proper and complete independent survey have served the owners better?