Hypothesis: Backpack is what's finally going to start pushing this card into relevance.
Analysis: There are two major issues with Relic Hunter that have always left it in the shadow of Charisma.
The first is that allies are just inherently more useful than most of the accessories. Holy Rosary sits alone atop it's throne for 0 XP accessories, whereas 0 XP allies includes powerful staples such as Dr. Milan Christopher, Peter Sylvestre, and even allies like Alyssa Graham or David Renfield that provide a flat stat boost along with a modest amount of damage/horror soak. Even Beat Cop (0) puts in big work, soaking one damage and one horror while providing the same +1 stat and while also having a useful sacrifice condition.
Those are just allies that provide at least one damage worth of soak and a +1 stat bonus too. That's not mentioning the more disposable ones like Art Student, Research Librarian, or even Arcane Initiate (0).
The second is that accessories haven't had quite the same support that allies have for a good while. Calling in Favors may not have been around for that long, but it's still a massive boon to Charisma decks. Again, even though we've had A Chance Encounter (both versions) for less time than Scavenging they're also just easier to use. The ally support also seems to consistently put the ally into play whereas item/accessory support doesn't.
Now however, we finally have Backpack. It doesn't dig as deep, nor does it put your accessories straight into play, but it does do something else important. It finds you multiple types of cards--and no, I don't just mean it finds you the types item and supply. It will help you dig up Flashlights, Lockpicks, or Magnifying Glasses so you can investigate, maybe help trigger that Scavenging easier? It can also help you dig up a weapon with which to defend yourself--and this is all in addition to helping dig for your accessories to put into play.
The only problem? We still don't quite have accessories on the same power level as we do allies. We don't have accessories with upgrade paths the way we do with allies. We don't even have anywhere close to the same pool of unique cards to choose from that aren't signature cards. There are only three accessories that offer any amount of horror soaking, and one of them costs a fair bit of XP. None of them offer damage soaking, which admittedly is fair from a flavour perspective (that's what body slot items should do). Even with how nuts Key of Ys is when powered up, you still defend it with allies. That card isn't that unfair compared to The Red-Gloved Man given the support that it has from the rest of the card pool.
Conclusion: We have the support we need for accessories in cards like Charles Ross, Esq., Backpack, Relic Hunter, and Scavenging. What we lack are the accessories that are worth supporting with these cards. Until that changes, accessories will be relegated to set and forget all-stars like Rabbit's Foot and Holy Rosary, with brief appearances from Cherished Keepsake or Elder Sign Amulet to help patch up the sanity of investigators vulnerable to horror.