I think it's primarily in the outer suburbs driving and parking is popular. For those who live more than 2km away from the train, walking is unlikely, and the buses are so hopeless that you can often drive just in the waiting time between buses (in theory you should be able to turn up for the departure time, but it could be up to 30 minutes late) (@Revenue, where is the bus tram-tracker?!)
Bicycle infrastructure is still mostly non-existent - we have poorly designed paths which don't connect, and lanes which force you into dangerous situations where drivers squeeze past, so most people won't consider bike until that changes in their area.
The key to increasing train stations' catchment size lies in the above - giving those people a credible alternative. When will Victoria take buses seriously? We put no thought into how our road network should work for buses - councils litter the street with bus obstacles and Vicroads is not much better.
In the inner city, residents should not be allowed to park on the street. There isn't enough space for this in our higher density future. We shouldn't be replacing rail commuter parking with resident parking, we should be replacing on-street car parking with space for bicycles, wide footpaths and street trees.