You could easily carry more arrows than javelins and even early bows had an accuracy advantage at range. Bows replaced slings, which were very effective but bows left sticks stuck through people when they didn't kill and disabling some is as good as killing them on a battlefield.

It wasn't all upsides since bows are more likely to be affected by moisture.

Javelins had their end times when shield walls stopped being a thing, the key function of a javelin was to bend on impact and make it arwkard to move.

Accuracy wasnt great but hitting a shield was easier than a person. Once people stopped using shield walls hitting someone with a javelin became much harder.

Arrows you can throw 500 into the air and a 25% hit rate will be seen as a success, you also do it from a greater range.

Enter the gun and you get even longer range and arguably better accuracy.

Guns and bows were used in parraell until guns gained the ability to be reloaded quickly - one step after muskets and one shot hand guns.