Misfires do not happen THAT often. And if they do, the audience is usually amazed too.
+Percussion weapons are boring. You lose the kick of having a huge explosion literally 2cm away from your face. Or getting sparks in the back of your neck from all the other dudes firing around you.
Or burn wounds on your hands and stuff. Exchanging a flint in the middle of the battle, cleaning your gun in the middle of the battle. You lose all that when using percussion. Pretty boring.
Most percussion weapons are also smaller and have a smaller caliber. Really boring, if you ask me.