Because Itachi knew he could not tell Sasuke the truth. Sasuke was still too weak.
Weakness was the original sin of all things, an unforgivable sin.
If he told Sasuke the truth—that the Uchiha clan had wanted to rebel and had therefore been purged by Konoha—then Sasuke would surely come to hate Konoha and stand against it. Konoha would never spare Sasuke's life.
Itachi did not want to see the younger brother he had personally protected lose his life like those clan members.
If that happened, everything he had resolutely done before would lose its meaning.
"Sasuke, it isn't that I don't want to tell you. I'm not telling you for your own good! Everything I did, I did for you."
"You don't need to know about those dark things. You only need to keep living in the light. You have such a good friend in Naruto, and you'll live a happier life. Then, in that happy life, you'll grow into a powerful ninja, restore the glory of the Uchiha clan, and protect Konoha's peace."
"As for those dark and filthy things, let me bear them. In your eyes, aren't I already a dark and filthy person?"
"Heh, this is good too. I'll carry my sins, this dark and filthy image, and be righteously killed by you, falling into boundless hell. Killing trash like me, I doubt Sasuke would feel any psychological burden."
In an instant, Itachi thought of many things. As for that trace of pain in his heart just now, he sealed it away once more, and his resolve became firm again.
Forcing down his emotions, Itachi still wore that mocking expression. He knew that expression would make Sasuke hate him, and that was exactly what he wanted.
"Hahaha... my foolish little brother, after five years, you still