Previously the FTC had been running such that, if it could not prove an acquisition was bad it would approve it.
In this case, back in 2012 Instagram was small, didn't charge money, and not considered a competitor to Facebook.
Well, we sure as hell know now it was a bad idea. Harmed consumer privacy, removed other competitors, and expanded the network effect of then Facebook.
And before anyone says "oh what about YouTube, X, TikTok, competitors" - Yes they should all be split off from their parent companies too.