It's because they are Playstation exclusive. It would be way too unfair if they had an amazing ability that x box cannot have. It's why hawk moon has a perk that can be found on quite a few guns, why Monte Carlo's perks are pretty useless and why the fourth horseman sucks.
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In addition to Luck in the Chamber, which a lot of guns can have, Hawkmoon has Holding Aces, which adds 2 more bonus damage shots to the magazine. No other weapon in the game has that Holding Aces perk.