The reason why there are so many Victorian teams (Collingwood, Carlton, Melbourne, St. Kilda, Richmond, Essendon, Western Bulldogs, Kangaroos, Hawthorn & Geelong) is because the Australian Football League (AFL) was originally the Victorian Football League (VFL) and became the AFL in 1990.
The 'interstate' sides joined as follows:
* In 1982 South Melbourne relocated up to Sydney to become the Swans.
* In 1987 Brisbane (Queensland) and West Coast (Western Australia) joined.
* In 1991 Adelaide (South Australia) joined.
* In 1995 Fremantle (Western Australia) joined.
* At the end of 1996 Fitzroy (Victoria) merged with Brisbane to become the Brisbane Lions.
* In 1997 Port Adelaide (South Australia) joined.
Regarding the supporter bases of each team, it varies and has anecdotally been regarded as 'tribal' just like in soccer. In Victoria kids can grow up supporting the team their father did but another kid might grow supporting another team for another reason. Rarely is support for a team in suburban Victoria geographical.
The support of the interstate teams is more clear-cut. For instance, in South Australia you're a Crows fan or a Power fan depending on prior allegiences (Port Adelaide was an evolution of a successful league side in the South Australian League).
Regarding support in Brisbane and Sydney, Aussie Rules is still behind Rugby Union and Rugby League in terms of media coverage and popularity but funnily enough the Lions and Swans seem to get higher crowds than 3 or 4 Rugby League games combined.
I hope this gives you a bit of info - some of the other guys on here will be able to extend on what I've said