Its kinda odd given all of the issues with the endgame how the two endgame options only have one vote apiece...
From what I understand, they're very similar. Maybe this is the reason?
Exo galactic revenge: "This would function somewhat like a CPA, in that a huge number of ships would spawn on the core homeworld and push through to the nearest player planet" And so on, to the player home world.
Zerg rush: "When the AI homeworld in question dies, it releases all mark I ships to attack you immediately, then after a couple of minutes all MkII" And so on, to the player home world.
Or maybe they're just not fun?
Neither excites me. In any game, I've already faced a billion hordes on my way to the AI homeworlds. What will facing a few more add? Besides, an end-game CPA when I've already won would suck
"You win!!" would become "You win... now DIE!!" My fleet can't be everywhere at once for a gigantic CPA/zerg at the end game and my turrets sure wouldn't be able to handle it.