Why did Imam Hussain (PBUH) depart Mecca without performing Hajj?

Answer

In the name of Allah: That was because Imam Hussain (PBUH) knew that Yazid (may Allah curse him) sent a group of Bani Omayyad people disguising as pilgrims to kill him (PBUH) even if he was in Kaaba. Therefore, he left Kaaba to preserve its sacredness. Allah knows best.