We can colonize the centre of Australia. There just isn't a reason to do so. You can see wherever they find a patch of minerals, a mining town springs up. Decades ago, there were towns all along the train lines to support the rail services, but as trains got more reliable they were no longer required. There are few advantages to being in the centre of Australia. Many of the people who live there do so because they have a connection with that land.

The average person would rather live in a crammed Sydney apartment complex then live in the middle of the Simpson desert.

Mars is new. We really don't know what will happen. There are a group of people throughout history that have "chanced it" and gone to the new world out of the hope they can build something better. You will have the same rush on Mars.