Another reason would be the volume of traffic (rail) and number of tracks to be crossed in the Metro area would have made the use of Gates almost impossible to operate.
However, once you were outside the Metro area or where there was little rail traffic and the topography didnt provide for either an underpass or a Bridge at minimum cost, then you had a set of gates.
I'm not sure what area is being mentioned here.
metro changes over the years ... Granville and west to Penrith had dual lines in the 1970's with the upgrade to quad line all the crossings (seven hills to Penrith ) where removed except on the Richmond line where it's a single. all flat earth areas. Richmond line is slowly being upgraded to dual track and the crossing are being removed with bridges or underpasses..