To be honest I think splitting a school between IB and VCE is a really bad idea. It creates segregation between the year 11s/year 12s (seriously, ask any school offering both and they'll report something like that), and it also splits resources, creating situations where schools have to cope with teaching two different systems and, as a result, two different types of materials. For those really curious, IB also doesn't really give you an advantage over VCE; all the top US universities don't look at either really (they just use the SATs), and other prestigious universities such as Cambridge and Oxford accept VCE. The latter also addresses Rohitpi's point - I don't see how the government could possibly see preventing schools from doing IB would keep private schools from providing International Accredition...
In general, you'd also be surprised at how powerful school councils can be, especially when they're supported by the donations of past students who like to cling on to tradition. At Melbourne Grammar in the 1980s/1990s, there was a push to make the school co-ed. However, the Old Melburnians (old students) said they would strip funding from the school if they did that, because they essentially didn't want things to be changed. Same principle might be why a lot of really prestigious schools with long traditions (eg. Melbourne High, MacRob) don't offer IB.