I'd have to think it didn't sell because it was expensive compared to its competition and worse on gas than its competition, while having lower power output. The H6 was great, but not a class leader for anything, really.
With that being said, I'd take the Tribeca's interior (5 seats) over the Forester's any day. Not sure why Subaru's parts-bin approach to its model line-up couldn't have shared more from the Tribeca than the Impreza...