One might well ask why "Hackneyed" has it's meaning of commonplace.. And if you do, you will see that Hackneyed comes from a "Hack" horse, the commonplace (often hired) horse you use for more ordinary riding tasks, you don't care if its a bit spavined or mouthed, it's not your best hunter.
In other words Hack -> Hackney -> Hack
A Hacking jacket likewise: its workday hard wearing cloth like Tweed. undied or dull dye.