Board Thread:Versus Debates/@comment-31568508-20170327125927/@comment-26369387-20170402012601

That's the thing: Kakashi thought that his Chidori was good enough to use in actual combat. After seeing Chidori, however, Minato saw it's biggest weakness: with too great speed, Kakashi couldn't react to Iwa shinobi's counter beacuse of tunnel effect, which is why Minato classified Chidori as incomplete technique at the time.

With Sharingan's heightened visual perception preventing the tunnel vision from occurring and its predictive capabilities making it easy for the Kakashi to avoid counterattacks, he could use Chidori safely and eventually upgrade it to Raikiri. Without Sharingan, Chidori is too risky technique to use.

It's explained pretty clearly in Kakashi Gaiden.