The problem is none of the alternatives offered a smooth UX transition. Mastodon is fragmented by design and Bluesky is gated to this day. There was never a true Digg-like event that caused user migration to reach critical mass. So people simply trickled back once the most volatile periods of post-Elon Twitter passed.
That doesn't change the fact post-Elon Twitter has severely degraded in terms of user experience (rate limits, blue check spam, API pay-wall, etc.) and Elon isn't doing the platform any favours by continuing to participate in detrimental ways (seen in the recent advertiser exodus).