Time more than difficulty.  Takes an expert - which most soldiers weren't - a minimum of 15 seconds to reload a unrifled musket, call it 3 rounds a minute for the experienced, average soldier.  (A rifled musket takes longer so call it 1.5 rounds/minute on average.)

Infantry were rigorously trained to hold their fire until there was a high chance of inflicting damage - with a smooth bore if a soldier actually hit what he was actually aiming at it was Pure Luck - with volley fire (throw enough lead downrange and you are sure to hit ... something?) and by that time they were too close to try and reload ... so it was on-in with the bayonet¹.  The Brits got so good at this they kept trying it up through World War One.  (Against machine guns and barbed wire.  (It didn't work so good.))

The usual canard bayonets were used to repel cavalry is nonsense.  To repel a frontal cavalry charge all infantry has to do is stand there.  Horses don't like running into a solid mass and will shy away.  Flank attacks can work which is why they were put on the flanks during a battle.  And which is why the British developed the British Square as a battle formation, if you don't have flanks they can't be attacked.

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¹  Despite what you see in movies most of the time the infantry on the defense heroically ran away when faced with a determined bayonet charge from close range.

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre

I have read somewhere that bayonet training is an important tool in breaking down the very common impulse not to harm a common human that you can see and identify with. Might be why they still have them.

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Bayonets are used both because of tradition and because soldiers need to lug around utility knives in any case. You might then just as well make it possible to mount them as bayonets.

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For a completely different and unrelated idea, maybe you could use bayonets when boar hunting? In Sweden we have just recently started using boar spears as a complement to knives/hunting machetes (while spears have been the modus operandi in France and Germany for like forever). But why lug around a spear when you can just mount bayonets? Many Swedish hunters use surplus military Mauser rifles anyway which readily accept bayonets.

The entire idea of using cold steel when hunting boar is that as boars are very tough, a badly aimed shot might just wound the boar instead of killing it more or less instantly. This makes the boar very angry and aggressive, and with barking dogs circling it at a short distance you don't dare fire again as the round might well hit a bone and ricochet, killing a dog. The thing to do then is to slit the boars throat with your knife while at the same time avoiding being disemboweled by the razor-sharp tusks. Our continental neighbors (who unlike us have hunted boars for more than the two decades they've existed in our fauna) long ago figured out that was very stupid, and as I said, use spears instead. But as most people around here don't have boar spears lying around, bayonets might well have a role to play.

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