That old chestnut. Why wouldn't they support the Swans already if they love footy so much??
a. They don't, for the most part, "love footy so much". That's the point of expansion, to give a large population a chance to come to love the game. It will take time and money, but it should eventually prove fruitful - and with minimal impact on the Swans, as the Swans don't have many fans out west. It also will allow the Swans to better target themselves, Sydney is a very big place for one team to try and cover with player appearances, etc.
b. (Only on anecdotal evidence) The Swans are seen by some as a plaything of the eastern suburbs. Some who otherwise may be prepared to give Australian Football a go aren't prepared to do so with the Swans.