Board Thread:Naruto Discussions/@comment-37594199-20181122223428/@comment-30323490-20181122224928

Couple things : He didn't awaken mangekyo straight after he got the sharingan. He awakened it couple years later (when Obito awakened it as well).

He never had eternal mangekyo sharigan. He only ever had mangekyo at best.

Also since it'snot his original eye, Kakashi can't deactivate his sharingan, hence being constantly drain by it. Itachi also stated that kakashi lacks the Uchiha body and their physical strength, followed by Kakashi saying that his body lacks the stamina for the sharingan.

Also, i'm pretty sure that EMS wouldn't solved Kakashi's stamina problems anyway, since he can barely handle the regular sharingan. EMS is just a solution for mangekyo sharingan drawbacks, which is a total different thing.