The game itself has some sort of limitation on banner slots, so you'd have to give up a number of clan banners to even get different strategus banners.
Well thats easy, the small factions like just traders, shouldnt get their own banner, but i mean Coalition stands for 3 Clans, with one banenr each, greys got some clans and randomers in them, mercs got 2 clans in them, druzhina, nis, templars also got 2 clans...
So .... its not an issue of space.