This was done as part of the "game balance" lunacy. It makes absolutely no sense, common, or otherwise. In the modern day, most US States restrict the use of X-bows for hunting game animals, explicitly because it is
sooo easy to use, and so deadly -- modern, compound X-bows, that is. In medieval times, the Papacy even outlawed their use in warfare, as it was too effective, too easy, too deadly! I believe Gary tried to use a "Disintegrate Spell" on that bit of history, but it made its Saving Throw, and so we still know about it.

A Sling is an under-valued weapon, IMO, but I play 2e rules, so I don't know what its stat's are in any other version. It has great Range, and with a lead Bullet, it does a fair amount of Damage when it hits. The ROF is slow, but it is still a decent, bludgeoning weapon, with excellent range.
As to the X-bow for a Wizard, I say, "Run with it -- it's your game, your rules!" If you find that it breaks the game balance, change it. I do not believe it will 'break' your game, in any way. Aside from all of that, look at what Gary did for the Wizard class, in the 1e Advanced Dungeons & Dragons rules... Cheers!