No reason, but I feel like if the trees starts to branch off it will start over-complicate the units and that kind of things really puts someone like me off. For example Warband's floris mod, I didn't play it because even the simplified version troop trees were to much to memorize. A longer troop tree would be less confusing than a branched tree.
Part of Floris' problem in that regard was the modmakers' insistence on both using non-English names and expanding the roster greatly. Yes, I understand it is a bit stale to encounter the same mix of descriptors -- heavy, lancer, veteran, archer, elite, champion, horseman, etc. -- but dumping wholly non-English, non-common unit names on unsuspecting players just added to the confusion and hurt the glance value.
(Of course, they then added the A2, H5, C4, I1 system to help, but I found that even more stale and not always terribly helpful since there could be two units of the same type, at the same tier, with far different capabilities.)
1257AD had the same issue at first, since they decided to use straight-up Latin for unit names. The English name subod was a must for me.
I still think they should just rip off system from Rus13.
It was like that:
You can hire pesants from vilages who can became ok warriors
You can hire militia warriors from castles or cities if owner is your friend
You can hire elites only from your own castle.
Any type has his own troop tree.
It was really cool and immersive.
1257AD had that. It was good.