1. Question: Why doesn’t the chosson break the plate by the tanaaim before a wedding, rather he only breaks the glass under the chupa, and the mothers of the chosson and kalla break the plate?
A. The Vilna Gaon says there is specifically a custom for a ceramic plate to be broken by the tanaaim because there is no foundation in the Torah for tanaaim to be broken just as ceramic can’t be fixed but a marriage has a basis in the Torah for being broken if need be, i.e. divorce therefore a glass is broken under the chupa since a glass can always be forged back together a new through fire symbolizing there is a halachic way to untie the knot of marriage, through divorce.
B. The Mishna Berura (560:2:9) says the custom to break these things both by the chupa and by the tanaaim is in order to mourn the destruction of the Beis Hamikdash and one should feel joy while still feeling trepidation.
Answer: Since the chosson’s full joy only comes when he completes the marriage under the chupa therefore he only has to break the glass to include mourning at that time and not before by the tanaaim. (See Dirshu Mishna Berura there footnote 20.)