Slings were a more effective, accurate, longer range weapon. The only longer range weapons of the day would be specific eastern bows. There were western longbows, but that's more or less solely a physical description. They weren't made for firing that long of range, and were probably either just a regional method of production, or they were used to fire a heavier type of arrow (such as the case of Rhaetic longbows; Rhaetic arrows are a bit heavier). Archers are included both in means of completeness and historicity though. Archers were used by barbarian peoples in levy, probably because it was quicker to levy hunters. Slingers in the mythological demi-history of Ireland are held in extremely high esteem. All the members of the Fianna were slingers, for example, and trained extensively in its use. Viewed realistically, it becomes apparent a trained slinger would be a valued soldier. His weapon hits accurately from a great distance and can outrange any of the bows many people would be familiar with. The same problem then would be time, and expense. A slinger would be a trained soldier (at least in the use of his sling), and could demand more treasure than a hunter with his bow who has no real training. Of course, also, training takes time, and, as mentioned, it'd just be quicker to levy archers from hunters.