The answer, of course, is the same one main stream geologists give for the same question:
why didn't geology create a grand canyon on every continent?
Because it was a result of local conditions not found anywhere else. The structure of the rocks, the lay of the land, the water flow patterns all were different and only in this one place did God in his glory choose to create such a singular monument to his splendour.
Actually mainstream geologists would probably leave off that last bit.