They wanted to dispel the suspicion that David was a rogue/assassin and used Death Attack on Goliath.
No, seriously, I don't see why they shouldn't have it. Maybe it is the way it's operated. While crossbows have an easy point-and-click interface, and bows only have to be drawn and aimed, you must swirl the sling around, which may be difficult when prone or in some other ambush position. But since the sling has no rules concerning firing from prone - at least none I'm aware of - and D&D usually eschews that much detail in favor of fun to play, I don't think why it should not be a rogue weapon. Let's await 3.5 and see what they did in that respect.
While not especially discreet in its use, the sling has the advantage of being easily concealed. It also won't get in your way while trying to sneak about. (Go ahead, try to move silently with a quiver full of arrows and a bow or xbow thwacking into everything.)
I house ruled the sling in, also a penalty to move and hide while carrying cumbersome weapons.
It would have made more sense, especially considering that the Sorcerer has all Simple weapons but Rogue doesn't.
It also would have saved them some text. Saying Simple Weapons, sap, short bow (normal and composite), and Short Sword is shorter than the current list. Medium sized rogues would know Rapier as well.
Guess that would have made too much sense and made the rules too simple.
As for why a rogue would want a sling, that is easy. As others have already pointed out, it is simple to conceal. In addition, it is easy to manufacture, and there is no problem finding ammunition for it. When you are going into an unfriendly area where obvious weapons would be a problem, a sling is a great weapon.
In any case, I was really hoping one of the original playtesters could answer the question. There had to be a reason the weapon was removed, but I can't really think of any.
Slings are actually surprisingly easy to use. They are as accurate as any thrown weapon -- with a little practice it is just another way to throw a rock. Certainly easier to use than a throwing dagger.
It does take significant skill to use a sling effectively in tight areas. But that is equally true of all large melee weapons, and some of the medium ones as well.