Because thankfully in Australia, there are unions that not only stand up for workers rights, but also for traditional family values. Many socially conservative unionists hold very high ranking positions within the party. If Labor decides to say "good riddance" to their very numerous anti gay marriage members, there will be internal bloodshed of the highest order. It won't go down too well with voters in some Labor areas either.
In "civilized" countries like Canada, the UK, the US, New Zealand - trade unions are in the forefront leading the charge in FAVOUR of ending discrimination against gays and lesbians. In Canada the Autoworkers and the Steelworkers were 100% pro-equal marriage before it was even fashionable and my understanding is that in the UK and the US organized labour tends to now be almost unanimously pushing for an end to discrimination and LGBT people. Why are unions in Australia being left in the dust? I have seen polls on public opinion in Australia on these issues and Australians themselves seem to be far more progressive on gay equality than are their politicians.
In the case of Julia Gillard, I assume that (like Barack Obama) she has probably been privately and personally 100% in favour of equal marriage her entire adult life. She probably was just too much of an opportunist to want to be a "trailblazer" on that issue and instead wanted to wait until there was more of a national consensus.