Fang might have a higher Poison chance, but Jab does more damage and Sharpedo doesn't have the luxury of trying to stall stuff out when it's so frail. It needs its opponents dead ASAP and consistently.
Plus two Poison targets (Tapu Fini and Clefable) are immune to Poison anyway due to their abilities.
It's true that Sharpedo should be trying to maximise its damage output to minimise the amounts of hits it takes, and poison jab does have the potential to hit slightly harder than poison fang .
However, running fang over jab is not about using Sharpedo solely to spread the poison status and stall, but to leave a lasting mark on certain walls (since toxic is far more threatening to them than both the standard poison and additional damage,and has better odds of landing the secondary effect).
Poison Fang and Poison jab are effectively as reliable as each other in bringing down tapu fini:
252+ Atk Mega Sharpedo
Poison Jab vs. 252 HP / 100 Def Tapu Fini: 88-104 (
49.7 - 58.7%) -- 67.2% chance to 2HKO after Leftovers recovery
252+ Atk Strong Jaw Mega Sharpedo
Poison Fang vs. 252 HP / 100 Def Tapu Fini: 82-98 (
46.3 - 55.3%) -- 16% chance to 2HKO after Leftovers recovery
There are certainly better odds in 2HKOing this particular defensive fini set,but this isn't even max defensive. Up against more defensive sets it's a 3HKO regardless of which move you run.
It's definitely one of the mons that sharpedo should have its team prepared to deal with instead,with the poison coverage move to be used as a last resort to break away that last 40% or so.
Bottom line: Sharpedo has poison coverage,which is useful, but should not be running the coverage to break down fairies/walls all by itself.