The Prime Minister's announcement of the policy included, unusually, a callout to one of the major news organisations (Newscorp) for their campaign (called Let Them Be Kids) in favour of such a ban[0]:
> I do want to pay tribute to the campaign that's been run by media organisations on this. I do want to single out NewsCorp for the campaign that they've run, Let Them Be Kids. It's an example of a media organisation being engaged in something that's of concern to the community.
[0]: https://pmtranscripts.pmc.gov.au/release/transcript-46363
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...
The motive is because it's easily gamed for propaganda. It's the easiest platform to bot, and thus exploit, for psychological manipulation.
Text is also the cheapest way to influence people - making videos is much more challenging.
And here I just called it more intelligent, lol. I should qualify that. I've seen her instagram feed too, and somehow it's even more absurd...