https://www.npr.org/2024/06/26/nx-s1-5003970/supreme-court-s...
I short: because it wasn't Facebook that brought the case.
Those next two sentences from your article seem to contradict your assertion that it was because it wasn't Facebook that brought the case.
But facts are facts right?
Zuckerberg did say Facebook was pressured by the Biden administration to censor covid misinformation, and the Hunter Biden laptop story [0], [1], [2] (multiple left-wing references for good measure). If Zuckerberg is telling the truth, that is a clear cut first amendment violation.
A private company can censor whatever it wants (mostly) but not at the behest of the government, there's law against that.
[0] https://www.pbs.org/newshour/politics/zuckerberg-says-the-wh...
[1] https://www.bbc.co.uk/news/articles/czxlpjlgdzjo
[2] https://www.theguardian.com/technology/article/2024/aug/27/m...