Weight and cost. Look at the [https://en.wikipedia.org/wiki/Brown_Bess](https://en.wikipedia.org/wiki/Brown_Bess) the British used from 1722 until 1838 its mass is 4.8 kg. When the fight reaches a short distance and you use the bayonet you would need to do that with the other muskets on your back.
Multiple firearms were something that was used. It was not uncommon for cavalry to carry two or even more pistols. That is in addition to a sword for close combat.
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