Because you can give 200 random guys a musket, teach them how to use it, and have an effective fighting force in a couple months. A bow takes years of strength building and technical training.

How early are we talking?

14th century small arms were terror weapons more than anything.

But by the 15th century, early guns did offer advantages over bows and crossbows, albeit not necessarily decisive ones. For one, they had a flatter ballistic trajectory than either - meaning aiming is much easier. Lethality means little if you don't hit the target, after all. Troops could also carry more ammunition, as a lead ball and powder take up much less room than even a crossbow bolt, and lead balls, for that matter, could be cast by even laymen with a few basic tools (since lead has a low melting point). And bullets had superior penetrative power in comparison to most non-English bows and probably even the majority of crossbows - important in a period when metal armor was widespread. On top of all of that, firearms require very little in the way of physical attributes to use.

Basically, you get a weapon that's as deadly as a crossbow, but easier to use, able to sustain fire for a longer period during battle, better against professionally-equipped troops, and terrifying to boot.

Every development from that point on increased the advantages of firearms, and decreased the disadvantages, until firearms were no longer 'competitive' with bows and crossbows, but explicitly out-competing them for military purposes.

The killing power of a bow depends on the archer's ability to draw it. An English longbow had a draw strength of at least 81 lbs:

Longbows were very difficult to master because the force required to deliver an arrow through the improving armour of medieval Europe was very high by modern standards. Although the draw weight of a typical English longbow is disputed, it was at least 81 pounds-force (360 newtons) and possibly more than 130 lbf (600 N). Considerable practice was required to produce the swift and effective combat shooting required. Skeletons of longbow archers are recognisably affected, with enlarged left arms and often osteophytes on left wrists, left shoulders and right fingers.

To kill someone with a bow like this basically required training from childhood, and produced substantial physical changes in the body. If you didn't train from childhood, your skeleton wouldn't develop structurally over the years to have the proper strength.

Along with the improving ability of gunfire to penetrate plate armour, it was the long training needed by longbowmen that eventually led to their being replaced by musketeers.

The killing power of a gun depends on the powder charge and the mass of the ammunition. It's not necessary for the troops to train for a decade or more to kill targets with muskets, even armored targets. You can train someone to load, aim & fire a musket in a day, and they could be fully trained to clean it, replace the flint, wrap their own powder charges, and cast their own musket balls within a week.

They were better because of logistics of training time.

Big scary noise. Imagine being a footman walking into battle, then hearing a sound like a mountain falling over, then a dozen men in your formation fall over dead while a cloud of smoke erupts from behind the enemy pike wall. I'd be pretty concerned.

But in terms of killing effectiveness, bows at the time were better.

Not OP but I would argue that in large scale warfare, how quickly you can get your troops to sufficiently handle their weapons is one of the most important qualifiers to rate a weapon. So early firearms were superior to bows because you could train your soldiers faster on them than on bows. And I think that is what the high IQ wojak means to represent, too.