G'day Tsubodai. Great question and while I am not the Mod creator I'm happy to explain exactly why.
Firstly it is important to realise how big the task of creating this mod is. It is not merely a add-on to the original game but a total conversion. What this means is that we can't compare this mod to others in the community. This mod will require: Adding damage attributes to every weapon so that it creates a logical balanced arrangement; Adding attributes to armour so that it represents a logical arrangement; Equipping every troop so that it is historically accurate; Determining the number of troops in each individual party; The balance of individual solidiers; The balance of combination of troops; The balance of nations; The balance of combinations of nations; The stats of troops. etc..
This is merely part of the task that this mod is undertaking. It is important to note that all these aspects are intimately related, a small change to one troop, as small as a change in damage value to a sword, can destroy the balance of every one of those aspects. Add to this that balance is not simple in Mount & Blade. We want swords men to be balanced to other swords men, but we want the same swords men to be balanced to spear men in a distinctly different way and we want this not in a group basis (like the total war series) but in an individual level.
It is important to note that due to the complexity of this task, as content is added the difficulty does not increase steadily but exponentially. If you have 2 factions, you have to deal with 1 lot of balance (the above task) to worry about. 3 factions and you have 3 lots, 4 factions and you have 6, 5 factions and you have 10. Now just think that this mod has 24 factions. 24!. In comparison, vanilla Mount & Blade has 6 and it some would argue that the balance of the game is broken. Which leads to the last (in this simplistic response) aspect, the mod needs to be fun. That is the biggest pressure. If the game isn't fun then the game is worth nothing, and all the work goes to waste.
So let's look at why we want to tackle this in smaller, minor beta mods. Instead of taking on the task above, lets focus on one faction, say Scythia Minor. We set up the scenes, we fill in the NPC's, we do the fluff, we add the weapons and Armour, and we throw in some spawn points for the surrounding nations. In this smaller environment we can acutely focus on the balance, and the interaction between the parties. We can set up region specific quests, fill in the scripts and make this region come alive. Finally, and here is the important bit, we can focus on making this region fun! Move on to the next region, the next faction, and make that fun. It is an exercise in starting small and expanding slowly.
The important point here is "Rome wasn't built in a day". Cities aren't made all at once. It starts as a small area, a small population, until the space is filled to a living standard and then it sprawls, growing from the small and turning agglutinatively into something great. Agglutinative is the key word.
I hope this answers your question. It is a tactic to create a large mod by starting with something small, balanced and fun and adding slowly and carefully into something great.
If you have any other questions let me know