Explanation: In the Renaissance period, the proliferation of firearms led to, initially, an 'arms race' between more powerful guns and more powerful armor. Guns won - rather quickly - as it became increasingly apparent that making more powerful guns was cheaper than making more elaborate and thick armor.
One of the great advantages of guns, other than their eventual capacity to overcome armor (as early guns were only middlingly effective against armor), was that you can train any idiot to use a gun with a few weeks of training; contrast this with years of training for an archer, a good cavalryman, etc. Crossbows previously filled this 'easy to use' role in militias, but guns have the added advantage of an even flatter trajectory, making aiming easier - or, in the case of volley fire, less necessary. Combine that with the terrifying smoke and thunder of a firearm increasing its ability to repel enemies, and you have yourself a weapon that quickly replaced the crossbow on the battlefield for both hardened professionals and hastily-gathered militia. Ensign Expendable is going to EARN his hardtack ration!
... it is probably not advisable to blow your only shot at that close a range, though. Use a dagger or something.