Simple: Facebook and Meta have garbage reputation compared to Instagram brand.
The public associates Facebook/Meta with excessive data collection, overhyped VR, intrusive tracking, etc. That association is much weaker with Instagram.
To be clear, I'm not saying that Instagram actually is any better than Meta/Facebook - but that their public perception isn't as disasterously tainted.
It's interesting. In media, Instagram is often portrayed as a device for young teens to feel insecure about themselves, leading to suicides and depressions, and the lack of any meaningful intervention by Meta/Instagram. Is that really a better public perception than privacy violations? Realistically looking at this, I doubt they'd get much of the Mastodon crowd, which is, grossly exaggerating, mostly tech and LGBTQ. So what remains is "normal" people who care about "normal" topics. I doubt these people care much about privacy (most of them are already on FB and Instagram).
The public associates Facebook/Meta with excessive data collection, overhyped VR, intrusive tracking, etc. That association is much weaker with Instagram.