But even if it was dominant my hunch is it was unlikely the only factor: Bluesky also rapidly outgrew Mastodon, without a Meta-like advantage.
But its true that mastodon did not have a major breakthrough as of late and bluesky will likely surpass it in the near future as some important "high information quality" communities (journalists, scientists etc.) seem to migrate there in preference.
Orientation towards general (mainstream, non-tech) users, easy usability etc is indeed a problem for the fediverse. The reasons are mostly an anti-commercial ideological stance which on the one hand makes funding scarce. Hence brilliant open source products - there are many more than peertube - remain unpolished, not marketed at all etc.). On the other hand this hostile culture keeps mainstream actors from joining the revolution.
But make no mistake this is a revolution. The hyper-concentration in social media is an aberration that does not fit any other pattern in society and the economy. Some more pragmatism from the decentralization pioneers will accelerate the inevitable.
[1] https://blog.cloudflare.com/radar-2024-year-in-review-intern...
To sum up my entirely unoriginal opinions:
1. Mastodon has far better usability than any other Fediverse software I'm aware of
2. Despite this, usability is still a material coefficient of drag on Mastodon's growth
To be clear, I don't believe Mastodon has to or even should aspire to match the growth of other more centralized networks; only that usability is a drag on what would otherwise be natural growth for Mastodon itself.
I know you and your team spend a considerable amount of time and energy on usability, so I hope I'm not saying anything you don't already know infinitely better than I.