Passenger volumes would have something to do with it - as I understand it, SYD is about 50% busier than MEL in terms of passenger traffic. Also SYD is obviously the most popular airport for tourists arriving in Australia. I am sure others with a deeper knowledge of this matter will offer other, perhaps better reasons.
Sydney is 100% more passengers than Melbourne, but Melbourne has also had capacity issues that should be resolved now.
Major International Airports (a)
Passengers Feb-10/% of Total
Sydney 895 188/44.2%
Melbourne 448 497/22.2%
Brisbane 304 459/15.0%
Perth 225 992/11.2%
The top 10 airlines make for some interesting reading, Air Asia appears to be the quiet achiever:
Share of passengers carried Feb-09 Feb-10
Qantas Airways ............... 22.9%.... 18.8%
Singapore Airlines............. 10.5%.... 9.8%
Jetstar............................. 7.1%.... 8.0%
Air New Zealand ................ 9.1%.... 7.8%
Emirates .......................... 7.4%.... 7.7%
Cathay Pacific Airways........ 6.1%.... 5.9%
Pacific Blue....................... 5.5%.... 5.9%
Thai Airways International ... 4.0%.... 3.6%
Malaysia Airlines ............... 3.3%.... 3.3%
AirAsia X ........................ 1.6%.... 3.1%