Because in any fight, bows will always be better than throwing weapons for a range unit. And I think that's the use of throwing weapons, a preemptive strike from the infantry. Nope, part of the beauty of crossbows is the low skill investment required to use them effectively (both historically and in the game). Before engaging to the fight and completely wrecking anyone, they already can deal damage to weaken the front line. Just look at the Huscarls, they have a one handed weapon, a shield, and a throwing weapon. However, throwing weapons are not meant to be used by a dedicated range unit. Plus, if the army somehow is still alive and reached the wall, crossbowmen are generally decent in melee combats and well-equipped with armors. Then you have crossbows for defensive sieges because their high damage combined with a decent number of xbowmen will destroy any army before reaching the walls (except the Huscarls, they're the mongols of Warband). You have bows for open field battle where you can use your shield wall to defend your back line. The way I see it, they all have their specific usage.
