When m0n0wall moved from FreeBSD 4.11 to 8.4, it brought greatly improved wireless and network card support, but that's it. Many other, more important things were a step back from the current FreeBSD 4.x. Network performance was anywhere from 10-20% of the speed it used to be on embedded platforms, and the memory footprint was larger.
Additionally, moving to a new image is a substantial amount of work. Many things work differently, and so backports and forward ports of older versions need to occur.
That said, work is being done on 10.1 as we speak. It will not be fast, but as support for network cards is needed, it will be ready.
TLDR: When it is ready. :)