From the introduction:
The crossbow is one of the earliest weapons most societies develop, generally shortly after gaining access to reliable metal production (often copper or bronze). It is also generally the first stocked weapon any given culture produces, and the first ranged weapon to store potential energy for later release without constant effort from the wielder. Though generally slower to fire and bulkier in design than comparable bows (and much bulkier than slings, though often with greater range), a crossbow is easy to learn the proper use of. Even after a military has access to early firearms, it often keeps crossbows in use for quite some time, and specialty uses of crossbows are sometimes maintained well into the era of developed firearms and even energy weapons.