You know that slings, as in weapon slings, were used historically against heavy-armored infantry, because when you got hit by an egg-sized ball of iron or lead, your helmet and skull get a nice dent in them? And things go even weirder when rather than ball-shaped, the projectile is in shape of two cones joined together at their bases.
So it makes a perfect sense that slings are so powerful. Their main issue was always about lacking accuracy, not the weapon being shit.
Either way, crossbowmen are usually low tier units, so they are cheap and easy to level up. And don't suffer (that much) from being set ablaze, like ballista does.