General story telling. There’s no point him getting a new sword if it doesn’t eventually make him stronger.
But it’s also the case that Shusui was brilliant sword in it’s own right- we aren’t talking about Yubashiri here, we’re talking about one of the only two black blades we know about in existence, famed trasure of Wano, sword of Ryuma, equal in rank to Enma. It was a damned good sword.
Enma has forced Zoro to develop in a way that Shusui couldn’t have done, and we also know that one day, Zoro can make it even better- a supreme grade in it’s own right. But that day hasn’t came yet. Right now, Zoro is still showing difficulty in using it. He would not have that difficulty if he was using Shusui right now- but he also wouldn’t have the opportunity for growth.
Enma is, pardon the pun, a double edged sword. He will get stronger with it, but until it’s mastered, Zoro’s disadvantaged in a way that he wouldn’t be with Shusui.