You have yet to prove that the FFG dice have caused any real harm in tournament games. Before FFG would consider changing the rule, there would have to be a solid case showing that people are at a disadvantage by using FFG dice compared other people. That the dice in question are having a controllable impact on the game.
FFG has a vested interest in seeing people use their dice, so you have to prove that allowing 3rd party dice would be a benefit to them by making the OP events better in some way.
When was it proven that the FFG template caused any real harm in tournament. FFG also have a vested interest in seeing people use their template. Also when was it provent the OP events were better in some way with third party template.
Yet we can use third party template.