They were going for supplies.

And they didn't fight for the map--not for the map alone!" His face flushed with new excitement.
"Perhaps I am wrong, but it all seems clear to me now," he continued.
"Ball and the two Frenchmen worked their find until they ran out of supplies.

Wabinosh House is over a hundred years old, and fifty years ago that was the nearest point where they could get more.

In some way it fell to the Frenchmen to go.

They had probably accumulated a hoard of gold, and before they left they murdered Ball.