The discovery of a way to produce gunpowder was quickly followed by the natural progression of guns. Guns — being metal devices capable of propelling metal shrapnel faster than the speed of sound — were something that could break through nearly any set of armor or magical defenses, and can be wielded by near-anyone without any formal training.
The only real drawback to guns is their difficulty in manufacture: each one takes considerable skill to construct. A mistake in the firing pin or barrel can cause misfires (often to catastrophic and deadly effect). That said, some are attempting to create ways to mass-produce these deadly weapons.