I suspect that it's more about the geometry of best fitting people into the space. It allows the right amount of room for centerpieces and bread bowls and such. At least as far as the caterer and wedding planner are concerned, conversation is the last thing on their minds.
Those tables are intended for eight comfortably. They stretch to ten. You can do twelve, but everyone is gonna be cramped.
It's a size and shape that's so omnipresent one expects that it's some combination of geometrical efficiency, ability to pass/reach things, some flexibility in self-organizing conversational groups, and probably some other things such as not being too large to setup, teardown, and store.
Those tables are intended for eight comfortably. They stretch to ten. You can do twelve, but everyone is gonna be cramped.