That sounds disappointing, however couldn't they just add these new factions from the faction replacer mods with the permission of the modder? They seem good enough too in my opinion.
The Warband Engine only permits 5 factions unless you do some serious tweaking. The other thing being that previous patches barely saw new units being added, not whole factions. If you really want to see this game get patched go donate a million euros and say that unless it gets patched, no money. I'm sure FSE would be more then happy to oblige.
The Warband engine can theoretically support an infinite number of factions, but it starts to get unstable. NW has a limit of five factions because of all the other stuff that the dlc does, and anything above five factions will start to lead into untested and possibly unstable waters.
It is very much possible to add new factions to NW, however this takes time as a fair few things need to be added to/tweaked in the scripts. This doesn't include the new uniform meshes, textures, voice recordings, weapon meshes, etc etc. So it's not really worth the time and effort required to add a new faction compared to the outcome.