1) It was never a cohesive tribe. Even the initial "boot-lite" of Sierra hurt. But there was never a huge group of 'insiders' - Erinn and Sierra were on the outs from quite early on, and the early boots weren't obvious outliers.
2) The Exile Alliance also fractured it, even though Brandon and Sierra didn't last too long. Prior to the merge they both stated in confessionals that their loyalties were elsewhere. They didn't even have any "doubt" confessionals, wondering which way to go. Instead, they were both ultra-confident that the EA was going to turn the game around. It did, indirectly, by diverting their loyalty away from their initial tribe.
3) The "Warrior Alliance." Even though Brandon was supposedly in it, Coach had Tyson and Debbie before the merge. This could have been the big alliance to go far in the game, but it didn't turn out that way because it was based on something other than the actual strategic game of Survivor.
So then comes the merge - where is the large alliance? In one sense, the Warrior Alliance, if it included Brandon, was in a perfect position. First boot the three Jalapaos, which is obvious. Then boot Erinn and Sierra, who were outnumbered. F4 was in the bag. But that wasn't good enough for Coach. He found out that Brandon had a HII, and instead of using this information down the line when it could be used to his benefit (perhaps when there were six left, and boot Brandon before Sierra or Erinn... a very nice blindside) he was insulted that Brandon hid this from him. His bizarre Warrior Code was broken, so the cancer must be excised. But he needed to find other warriors to take brandon's place. JT and Stephen were more than willing - getting rid of ANY Timbura, especially a strong one like Brandon, was good for them. And at that point Timbira was doomed.
I don't blame Coach entirely. Tyson and Debbie should have taken him aside and done a Chris on him: Yes, Brandon lied. He must be removed. Just...not...yet. Maybe they did. But to replace Brandon, whose loyalty was somewhat under suspicion, with two people who logically had zero loyalty to them, was a very stupid decision - warrior ethos be damned.
So why did Timbira fail? Because Timbira as a cohesive unit never existed.
edited for clarity











