Why do all tribes mourn at Christ's return?

The mourning of all tribes at Christ's return is a direct consequence of their realization of His true nature and their past unbelief. Revelation 1:7 indicates that even those who pierced Him will see Him and mourn. This is echoed in Zechariah 12:10, where God promises to pour out a spirit of grace and supplication leading to mourning over their rejection. The significant implication is that when faced with the reality of Christ's authority and judgment, all will recognize their culpability, leading to deep mourning for their sins and rejection of Him.