Why does Amir tell anybody about what happened to Hassan in the alley?

Amir does not tell anyone what happened to Hassan in the alley because he failed to step in and defend his friend. He ran because he was afraid, he was unwilling to help even though he knew if the circumstances were reversed, Hassan would jump in. Amir ran because he wanted Baba all to himself, he wanted to return to the pride he knew his father would exhibit after the kite running. Thus, he sacrificed his friend and justified it because Hassan was only a Hazzara.