It was discussed that T6 was not meant for current Rancor Raid, and that future ways of earning them would come in future content.
There were also some 6* and 7* mods released via rewards from completing the Rancor Raid. We are removing those from the Rancor Raid, lowering the rarity of the 6* and 7* mods acquired down to a high powered equivalent of a 5*, and are working on new ways for players to earn those items.
https://forums.galaxy-of-heroes.starwars.ea.com/discussion/55700/game-update-7-15-2016#latest