Not a historian, but many years experience with muzzle loading firearms. Lead has many properties that make it desirable for bullets.

First it has been available commercially for many centuries, dating back to the Romans, at least.

Next it is easy to work with, melts easily in a simple pot over a wood fire, no forced draft in a specialized furnace such as would be required for iron or copper alloys. And can be molded into shape in a simple mold

And finally from a ballistics point of view, its heavy. Thus a small chunk can carry a lot of energy. Granite for example has a specific gravity of 2.6 While lead is 11.3 In other words a given chunk of lead is better than 4 times heavier than a rock of the same size.

So it comes down to ready availability, easy to work with and effective.

This question is more suitable for /r/askscience.