TW has to worry about the game balance and flow, which expanded smaller factions could so easily throw off completely. That, and fleshing out all the minor factions in a way that they'd work as a major faction could take more work than it may seem initially.
But I like the idea, especially because:
I'd love to be a son of the forest and take my brothers and sisters all the way to a kingd... uh, a community that has borders and sort-of-a-king-but-not-really-a-king-because-everyones-equally-important-among-the-sons-of-the-forest.
I hope some modder goes on to flesh this idea out if it is not in the base game.