Low damage die, short range compared to other projectile weapons, a move action to load it so you can't use rapid shot or iteratives without a feat tax - which feat you could spend getting proficiency in a better weapon instead. Is it hard to see?
Low damage die - Irrelevant, most of a character's damage comes from static mods and the sling uses the strength mod making it a solid choice for a character lacking bow proficiency who has a decent strength score.
short range - A valid complaint but the effective range is long enough to be useful most of the time as most DMs won't let you pepper an enemy with 4 round of longbow fire at max range at least not in my experience ymmv.
Move action reload - Same as a crossbow which has the same proficiency level and costs money on top of that, on top of which this is only an issue if you have multiple attacks and are therefore a ranged attacker primarily or higher than level 6 in which case you should have just got a bow or used spells if you lacked bow proficiency.