No nails screws or dowel pegs have been invented. Any wooden object has to be carved from a single piece of wood. Do you have wheels? Do you need them? mud roads or slippery rocks laid out on roads make pulling skids by animal possible without moving parts like axles to make wheels. This would mean the trigger system for the bows would be impossible.
If you have wheels, the trees for the axles have to be huge to survive the stress, because trees on your world are flexible and weak (making regular bows possible) but small axles for triggers and other mechanical parts are too brittle. Only by making them too large to be handheld would they be possible. No one wants to build a crossbow that takes 6 horses to haul... they and it would sink into the mud.
Anyone seeing a giant machine pulled by six horses would have time to move away before they could aim at you, making it very impractical. If it is to be used against a stationary castle, the fields surrounding the castle can be flooded, making the mud impassible for miles around, like a swamp. 658
The crossbow might even float away in the current. Trenches or even tree roots could slow its journey towards you. Horses could be slaughtered or poisoned in the night by spies leaving humans to pull their awkward invention themselves. The monster contraption would have to fire whole trees as arrows, and would take 3-4 hours to load in between shots. Less range than a jousting field (Is it called a "List"?)
It would become an epic tale of failure sang by the bards for generations, a combination of <PERSON> and <PERSON>'s Last Stand. "The Tale of the Too-Big Bow" would be told around the campfires of warriors for 30 generations, with characters making sound effects like The Three Stooges. It would become the Anti-Tale of The Trojan Horse. People would laugh at you less if you glued feathers to yourself and jumped off a cliff trying to fly. (only to be eaten by a giant bird before you hit the ground.) 342