Because based on the earth being a large ball, people had theorized that there had to be a large land mass down there somewhere, in order to balance the entire weight of the planet. Seriously. And everyone generally believed this theory, so everyone wanted to be the first to find and discover this new land to obtain fame and fortune.
Terra Australis Incognita means "The unknown land of the south.". What makes it unique in history and geography is that the land was hypothesized as existing long before it was ever actually discovered, based on nothing more than a concocted global equal weighting theory. Early explorers did a good job mapping the northern hemisphere, and once they learned and all agreed the world was a round ball, they theorized the weight of all the land in the northern hemisphere must be offset or balanced somehow by an equal (but as yet undiscovered and un-mapped) amount of land "somewhere" in the southern hemisphere. They assumed that if this wasn't true, then this big ball called earth would be top heavy and flipped around. So off the explorers went to find the missing large chunks of land that they "knew" had to be down there somewhere. And find it, they did.
