Breezy,
I believe the main reason there is no main deck fire suppression is that it hasn't been required by regulation.
FedEx is installing a self designed main deck fire suppression on their MD-11 freighters. It's a system that uses a can penetrator that runs along a track along the ceiling of the aircraft and is completely automated. It doesn't even have a system test or indication in the cockpit to show whether it's operational.