I am not sure why Reddit was excluded, but the implied reason for the view I outlined in my comment is not about preferential treatment for one of the mainstream media firms so much as about a tool to drive a return in advertising dollars to traditional media, and away from technology firms. There is also more explicit work being done to do this.[0]

Having said all of that, the social media ban polled well and I think that the popularity of the policy is the parsimonious explanation for why it was developed.

[0]: https://www.afr.com/politics/federal/labor-finalises-tech-gi...