This is quite interesting.

I do not think this is one of those situations where there is a single reason to explain it but several reasons. One no doubt is metallurgy and foundry. Aside from having the idea of rifling produce it is a different matter. Rifling the interior of a cannon requires certain degree of accuracy and certain technology that would not be available until the Industrial Revolution in the 19th century, or at least it would have not been sufficiently reliable until them.

Another thing to consider is physics. The arrow was probably the only similar shape projectile around and as a matter of fact machines existed since the Middle Ages to throw big heavy arrows again fortification, however, the natural shape to plug the cannon is a sphere. These were in origin stones, not necessarily foundry as most may think. So it's all about blocking the whole to generate the required pressure to produce impulse, a sphere here is the most handy.

A smooth-bore cannon (it's proper name) , or musket, has not got a terribly shorter range than that of a rifled one but it is FAR MORE inaccurate shooting at the same distance. An American Civil War example, a smooth bore musket may be effective up to 50 meters while a rifled one can go up to 300 meters, sometimes more. The problem is that the projectile not having the rifled grooves inside the bore would rebound inside the bore as it traveled it s length not necessarily coming out aligned with the weapon axis. This does not explain the shape of the projectile but serves as an introduction to artillery and infantry tactics which PLAYS a role.

On to Tactics them. I will leave aside the siege methods as have less importance in this discussion. Artillery tactics did not see much of change for centuries until the coming of Napoleon, who being an artillery officer himself began to mass it against infantry formations, until them artillery pieces were used spread all along the front. Now infantry up to the American Civil War, in Europe changed a bit earlier, used the tactics of the line. This is square infantry formations several ranks deep advancing shoulder to should until physical contact with the enemy line. Here is where a conical accurate projectile as matter of fact is not desiderable. Why? Well, you can aim a cannon (between 75mm to 120 mm in caliber, they were in reality measured in pounds as a reference to the weight of the projectile, a Civil War 12 pounder would be aprox 4.62" or 117 mm) to a soldier in a formation and may be blow his head and may that of the guy behind him if you get them in a straight line, but that's about it before plunging into earth. A round shot by contrast would do that and then would go bouncing around severing legs, arms and God knows what sort of damage in a tightly packed group of soldiers; even if you fall short it still may bounce on to the soldiers formation, a conical projectile via contrast would just hit the ground and sink The advantage of accuracy and greater range would come with explosive shells, so you can inflict damage at greater range before they close in, so not necessarily related to shape but projectile technology, up to them most would use solid shot. Last but not least a smooth bore let you use other sort of ammo, for instance canister for close range so the smooth bore becomes more useful.

I hope this helps in some degree, Ian Hogg has a number of good books on artillery history to start with the basics.