actually it was both the depletion of suitable wood producing trees such as yew and the decline in the amount of people who physically condition and trained to use a warbow to its maximum effectiveness. There was a phrase about when training a longbowman you start with his grandfather, as it pretty much took a lifetime to master the bow and learn the required skills to pass down to the next generation. Training firearm equipped troops on the other hand took only a few weeks with basic instructions. While a musket or crossbow were better at penetrating armour, a longbow in the hands of a trained yeoman had a greater rate of fire and was more accurate at long range.