Once he is trapped within Itachi’s illusion, there’s no way for Naruto to fight back. So Itachi uses this chance to plant one of his greatest weapons inside Naruto: Shishui’s Mangekyo Sharingan.

Even when he died, he still didn't reveal the full extent ... While Naruto spends his time getting stronger to fight bad guys, redeeming some along the way, Itachi doesn't have it as easy.