Zoro doesn't give two shits that Shusui is an important blade for the Country.
It wasn't until after Zoro saw what Enma could do that he accepted the trade.
And what the hell are you even comparing Devil Fruits to Enma for? They do not function the same.
Right now, Enma is serving to help Zoro become stronger. Once Zoro is used to the Sword, it will be all Zoro. But with that last attack, Enma is what contributed to Zoro using that much Haki in a single attack.
Again, Big Mom noticed the Sword was special, Kaido noticed the Sword was special, Zoro noticed the Sword was special, Kinemon knows the Sword is special. We've seen what Enma is capable of doing, but for some reason you Zoro fans can't seem to wrap your head around this.