The main reason in general for running an AV is a bulky switch in to sponge special attacks. Tangrowth and Tornadus were two of the most popular and successful AV users and that's because they acted as pivots with Regenerator to replenish their HP. While Tornadus is also SR weak, Regenerator and its speed offset this. If you weren't going to act as a pivot, it was demonstrated that the longer you intended on staying on the field, the bigger the payoff of running Leftovers over AV became, as Leftovers alleviates passive damage as well, not just special attacks. For every turn you stay in with Araquanid and are not getting hit with a special attack, that's a turn you could of had Leftovers recovery. For most cases in the longrun, AV does not benefit enough over Leftovers. Araquanid does have great attacks to work with, but its low HP means entry hazards are a far more significant threat most of the time, because when they are on the field you will always take damage from them. Tornadus was SR weak too, but it had a Spikes immunity unlike Araquanid, great speed, and Regenerator.
+1 252 SpA Adaptability Porygon-Z Shadow Ball vs. 252 HP / 0 SpD Assault Vest Araquanid: 76-90 (43.4 - 51.4%) -- guaranteed 2HKO after Stealth Rock
+1 252 SpA Adaptability Porygon-Z Shadow Ball vs. 252 HP / 0 SpD Araquanid: 112-132 (64 - 75.4%) -- 6.3% chance to OHKO after Stealth Rock
Sure, strong special attacks may hurt a lot less but how many are you honestly switching into and reducing the hits required to KO? With Araquanid's speed and no priority or Regenerator and massive hazard weaknesses, it honestly does not see the benefit from the item that is intended for it.
But hey I'm reasonable. First off, moderators can request I add it if they think its good for reasons I'm missing. Otherwise, show an AV set and say what it can do that other sets or mon can't do better.
EDIT: Actually the best comparison to Araquanid is probably Dragalge. Their stat spreads are virtually identical:
Araquanid: 68/70/92/50/132/42
Dragalge: 65/75/90/97/123/44
Both a slow and specially bulky with mediocre offenses but with an ability that dramatically boosts one or both of their STABs. Bot have good defensive typing and Araquanid is immune to burn while Dragalge immune to Poison, and neither have access to stat boosting moves. Assault Vest was never a recommended item on Dragalge, even if it would hypothetically make a better one than Araquanid having no SR weakness, Scald and Sludge Bomb to spread status, and Dragon Tail for phazing. Yet, I don't even remember it being humored as a good set at the time iirc.