While we're looking at the differences between Sydney and Melbourne scenes (and we're talking a fair bit of guesswork here) there was another factor that came into play as well.
While both cities enjoyed bayside or beachside locations the presence of Sydney Harbour, a large body of water with few crossing points probably made the development of a city-wide scene difficult. Sure, there were probably quite vibrant little scenes in places like Bondi, Manly, Paddington, and, particularly Kings Cross but I suspect that the first two weren't all that easy to get to unless you lived in the vicinity.
In Melbourne, on the other hand, it was relatively easy to travel from one side of suburbia to the other. Stephen Cummings in his Will It Be Funny Tomorrow, Billy, writes of roaming Melbourne from one side to the other when I was twelve, checking out groups I read about in the teen music rag Go-Set (p. 4) and recalls that every church, community hall and police boys' club from Rosebud to Altona had some kind of beat group playing (p. 5).
Now, the same thing may well have been true in Sydney, but I suspect people were disinclined to travel too far away from their home turf or the inner city public transport hub.
The ease of getting around Melbourne more than likely contributed to the musical scene as well. A series of reminiscences in Rhythms magazine by ex-Campact man Keith Glass talked about playing sets in two or three (maybe even four) suburban dances on a single night, something I suspect would have been a little more difficult in the Harbour City.
But Sydney and Melbourne was where you needed to be if you were going to hit the big time, and the big questions for up and coming acts were (a) how to get there and (b) how to find enough work to earn a living when you did.
On the second count, the number of suburban dances in Melbourne probably helped, and it's interesting to note the number of ex-Adelaide acts that shot to prominence after relocating to Melbourne, with Shorrock's Twilights and the Masters Apprentices being two prime examples.