What makes John link the Passover prohibition of breaking bones (Exodus 12:46) with the Zechariah 12 prophecy that “they shall look on the one whom they pierced” in John 19:36-37?

One of the links is Passover itself. “Not a bone shall be broken” is clearly a Passover text, but so is Zechariah 12, the mourning in each household resembling the mourning of the Egyptians at Passover.

John witnesses an event that is already testified by two witnesses, the double witness of Exodus 12 and Zechariah 12. Both texts, fulfills in Jesus, demonstrate that He is what John the Baptist said He was: The Lamb of God come to take away the sins of the world.