[dmm]

Remember that during the heyday of longbows, the English army was small, even by standards of the day. For example, the French had them way outnumbered at all three decisive battles of the 100 Years War (Crecy, Poitiers, and Agincourt). Perhaps this was because the English army in all 3 battles was only an "expeditionary force" or perhaps they could never muster large numbers of trained longbowmen.
Anyway, by the Napoleonic era, armies were much larger. Agriculture had undergone a revolution, freeing up a lot of manpower for warfare. Plus, more of that manpower lived in urban areas where there was less opportunity for practicing archery. Plus, governments began relying more on conscription and the "citizen soldier," especially during times of crisis. Lastly, the generals were a bunch of retards who used up the lives of the foot-soldiers like they were squirrels. So the government didn't want to spend large amounts of money training them. (In fact, some governments put criminals into the army, perhaps explaining somewhat the generals' attitude toward footsoldiers.)
Oh, five more things: 1) muskets are easier to use in trenches and forts and lying down; 2) you can put bayonets on them; 3) you can have several muskets loaded and ready, so the initial rate of fire can be quite high; 4) non-combatants can load the muskets behind the lines, again upping your rate of fire; and 5) gunpowder and lead can be used as ammo for muskets, pistols, rifles, cannons, and petards.