This is mostly hearsay and some conjecture on my part, but this is how I understand the history.
There was a thread at TOP where the person claimed he was the lead engineer for the AX8 and FX8. He explained that he decided to port the code from the Axe-FX II firmware to these new products in the way that made most sense to him rather than in the way that was consistent with how Cliff had coded the Axe-FX firmware. He explained that he felt it was necessary to take a different approach due to the differences in the core DSP. It was his opinion that doing things Cliff's way was needlessly complicated and that it would have made the products worse. Cliff was not pleased with this choice and they parted ways sooner than was anticipated. That ended up leaving the AX8 and FX8 in a bit of an orphaned state.
I suspect that part of the reason why it takes so long for updates to get ported to the FM3 and FM9 is that the project teams are required to do things in a manner that is consistent with the Axe-FX III firmware code. Although it takes longer, it allows for nearly seamless interoperability of patches between devices and it will mitigate mess caused by a firmware engineer leaving unexpectedly.
Again, this is just what I remember reading on an internet forum years ago from a guy who claimed to have inside knowledge. It may be totally untrue.
Edit: See Fractal Audio's response later in this thread.