Uruk-Hai Crossbows

Uruk-Hai Crossbows or Crossbowmen are common ranged class for Isengard
After Scout bow this Crossbow even pierces the armour better than the most powerful bow in Arda.

The crossbow is a ranged weapon consisting of a bow mounted on a wooden stock. The string, along with its missile (referred to as a bolt), is drawn and then held taut by a mechanism featured on the weapon's stock until it is fired by a trigger lever.

Why is this ranged unit more common than Scouts for ranged instead?
wielding powerful crossbows that, once loaded, are easy to aim and fire and can easily penetrate all but the strongest armour. This enables them to take out high value targets while their natural resilience and thick plate armour allow them to do so under sustained attack as the Uruk army advances on enemy positions.