I think Garrosh got the spot by default. Grom is a fan favorite so of course we want him to be the Legendary creature. He is the ultimate warrior, say what you want about Broxxigar if Grom were in the same position he'd do the same. Orgrim is also a fan favorite but the problem lies in the equipment. Thrall wields the Doomhammer now and there is no reason why Orgrim Doomhammer would wield anything else. Having a warrior class without a named weapon is just wrong.
As this is the case they should have made Grom Hellscream the warrior. Who wouldn't love to swinging in with Gorehowl as him. Then they could have made Orgrim Doomhammer the legendary creature. They could even give him a deathrattle ability where Grom get's armor stacks.