There was quite a bit of consideration done with regard to which factons would be made/not made. Ultimately, we had to have 2 things to make a faction 'viable'

1) It needed to fit onto an area of map that we could sensibly make

2) It had to have enough units/data to make it a viable and buildable faction

We also had to consider just how much work is needed to make a complete faction, with all the models, skins etc. that are required to make it look good. This was why we decided to limit to total number of factions in the initial release. If we tried to do them all right off the bat, we would end up with nothing finished!

We figured we could always release add-on races where the map allowed, or make individual special units from the excluded factions to bring in as mercenaries or rebel units.

It's a balancing act, and there are bound to be factions that people want that we are not planning to make..... but we had to start somewhere... and the biggest factions are the ones we started with.
Once we have done those, we can think about other additions, like Kislev, wood elves and suchlike.