Say, for same reason Ace's execution were to take place on of the world government's islands. Imagine if marineford arc was switched to Erness Lobby. How do you propose would Whitebeard have gotten all his allies + his own crew to infiltrate Erness Lobby?
I want details people. Oh, and remember how at the end of the war Blackbeard went crazy and created heaps of earthquakes. I don't think Erness Lobby could've taken it Lol. Its a island in the sky with a waterfall on the side. Breaking the standing ground would mean death for many pirates and marines.
Yeah, how would the battle have gone then?