I agree with the above posters that they were too pricy and that the styling, inside and out, didn't help matters. However, the biggest reason is that the vehicles just didn't match the demographics of the typical Subaru owner. And most other potential purchasers of the Tribeca migrated to more popular car makers/car lines.
I was still working at a dealership group that had Subaru, Toyota, Nissan, Kia, etc. Just about all the Asian makes except for Honda. When the Tribeca was produced, I was a former service manager at this dealer group who had semi-retired and was rotating around to all of the seven different store's service departments to fill in for vacations, trainings, firings, etc. I would see many potential Tribeca buyers shopping at Subaru but then see them later at one of our other stores with one of the other makes. Bottom line, the Tribeca didn't appeal to traditional Subie owners and there was
way too much competition with other maker's SUV's and Crossovers for the Tribeca to make any headway in the marketplace.