I suppose it's because the Haversack puts the wanted item on top? Maybe the intent is that when you retrieve an item from the Bag, you get a random item, and you have to dig through the whole bag to be sure of finding what you want...
...yeah, I know, I'm reaching here. Reading the rules for the Bag in isolation, the obvious conclusion is that you get the item you want with 1 action.
The only other advantage I can see for the Haversack is that it weighs 5 pounds instead of 15. Since the whole point of such items is to save weight, that does matter. But 10 pounds is not enough to justify a whole rarity level IMO, especially given the Bag's superior capacity. Maybe if the Bag were 25 or 30 pounds, it'd be worth it.