Moorabbin had 3 airlines at one point servicing Tasmania and a variety of other routes. Only one remains which pretty much only does King Island now, lack of airport upgrades and suitable replacement aircraft slowly sent them all broke bar the one. For a secondary airport to be successful it has to be either much cheaper than the incumbent or similar price and much more convenient. Avalon was same price to slightly cheaper and less convenient, Moorabbin was more expensive but slightly more convenient. Then you have to climb the mountain of Australias brand loyalty mentality, which is slowly wearing down, but still a major factor in choice of airline. Then you have other factors like connections to other services and such, which comes into the convenience area. Regional airlines must have low transit time for passengers to catch domestic and international services and so on.