Why would he have already received it? He has three swords at the moment. Shusui was traded with Enma, which was a reason for him to get it at the time he did. It's almost certain he will get Nidai Kitetsu by the arcs end, and in order for that to happen, Sandai needs to break, which would put him down a sword, and in need for a quick replacement, where Ame would come into play.
And how do we know Ame can be wielded by anyone? It's a sword made by Hitetsu, that Oden used for years. If there is a gimmick to the Sword (It's a cursed Sword), which most likely would be the case, then Zoro, someone who is already used to wielding a Hitetsu made sword, would be the best candidate.
It Enma means to cut through Hell, that means it's a "light" based Sword, while Ame which can slice through heaven, would be a dark/cursed Sword. Which fits with the type of Swords Hitetsu is known for making.
And how do we know Ame can be wielded by anyone? It's a sword made by Hitetsu, that Oden used for years. If there is a gimmick to the Sword (It's a cursed Sword), which most likely would be the case, then Zoro, someone who is already used to wielding a Hitetsu made sword, would be the best candidate.
It Enma means to cut through Hell, that means it's a "light" based Sword, while Ame which can slice through heaven, would be a dark/cursed Sword. Which fits with the type of Swords Hitetsu is known for making.
