It just seems like an unnecessary extra distance to go. Why didn't the northern/northwestern part of it (way closer to pretty much everything) become the center of colonization?
For the very simple reason that most of Australia's fertile land lies in the southeast/east of the country. Botany Bay, which today is the Sydney region, is one of the most fertile areas in Australia, whereas the west and northwest of the country is extremely dry and is sparsely inhabited to this day.
Because the South-East of the country has the climate closest to that of the UK, and so would have been the most desirable land for Anglo-Celtic colonists.