The amazing thing is that numerous sources state that slings had a rate of fire at least as fast as bows, and a range that likely exceeded them, with a heavier and faster projectile, that was possibly more accurate and more damaging. How can this be? Of course, you also have the case of the crossbow with clearly deficient rate of fire, and thus borderline useless in standard D&D play. And yet at a late point it was favored over the bow. Why?
Among other resources, I came across a very nice article by Chris Harrison (Assistant Professor at Carnegie Mellon Human-Computer Interaction Institute), originally published in The Bulletin of Primitive Technology (2006). Near the conclusion he writes:
When looking at the evolution of ranged weapons, there is a trend towards increasingly simple operation. The sling requires enormous skill, one that can generally only be obtained with training from childhood (Hawkins, 1847; Korfmann, 1973; Wise, 1976; Ferrill, 1985). Without this mastery, a person armed with the weapon would be practically useless. The sling is exceptionally difficult to aim because it is being rotated when fired. It is common for people to fire projectiles backwards when they are first learning, meaning a high degree of proficiency is needed before they can be safely placed in a battlefield situation. On the other hand, the bow could be taught at any point in life, and be deadly with minimal experience. The bow does not suffer from the sling’s accuracy problems because of its ability to be drawn and then aimed. However, archers did have to be strong, which increased the required training time (Wise, 1976). The development of the crossbow with a mechanical device to cock the weapon enabled anyone to use it and have the ability to kill even an armored soldier at distance. The crossbow was the first true ‘point-and-shoot’ weapon, as it could be cocked and then easily aimed using the large stock. Although much slower to reload than bows, it was seen as an acceptable tradeoff for the ease-of-use gained. The shift to firearms was similar. They were even slower than the already sluggish crossbow, at least at first. However, the operation was simple and there was no physical strength needed to load the weapon. Also, its ‘point-and-shoot’ nature made someone with almost no experience immediately useful on the battlefield, and very deadly. This evolution occurred primarily because of changes in military and governmental organization. In feudal times, lords could recruit their serf population as soldiers (Wise, 1976). Many of these men were already proficient with the bow or sling, which were used for hunting game. However, by the High Middle Ages, nations and cities had developed large standing armies, which were recruited, sustained, and equipped by the government (Martin, 1968). An increasing number of these recruits were from urban populations which had far less exposure to ranged weapons. These units had to be trained from scratch and there was a high turnover. This led to the increased use of weapons that were deadlier with less training. The sling was perhaps the least effective choice of ranged weapon in this role.
See the full article here. We might phrase that observation in economic terms; the advance of missile weapons was to get cheaper and faster -- lower quality, but able to be fielded in larger numbers and in that way more powerful. (We might well look to many things in our own era that became dominant by virtue of being crappier but cheaper: MP3 audio encoding, travel agencies, car ride services, etc.) Gygax actually makes much the same observation in an article in Strategic Review #7, "The Missile Weapon in Classic Warfare". (!)
That said, this is not something that D&D models very well. By default Fighters simply have proficiency in all weapon types. If the sling were the best missile weapon in an expert's hands, then every Fighter would be carrying slings and nothing else. If we want our game to look like the medieval era that would be a bit jarring. In any heroic story, the protagonist uses the weapon common to his people; e.g., David with his sling, Robin Hood with his longbow, William Tell with his crossbow, etc.
So in lieu of modelling more ancient/personalized weapons as taking greater amounts of training time and commitment, AD&D reverses the statistics to make sure that slings are a deficient choice for adventurers. What could we do to give slings their real-life fire rate and range (and advantages in small size, low weight, conceal-ability, etc.) without their completely eliminating the use of bows by PC adventurers? Or would that actually be acceptable?