Maybe they don't want to use Hunter Stadium because it's set up for rugby league and they don't see the need to use it for a one off football game in August that's not going to attract a large crowd?
I guess these early rounds are seen as pre-season games and of not much importance to A-League clubs in the scheme of the overall season so they're using them to 'grow the brand'? Personally I see them as hugely important - knock out football is exciting and would hope clubs approach it as something that fans want to see taken seriously.
I for one would be a little disappointed if the Phoenix didn't take our FFA Cup game as a serious game.