I'm moderately active on X, and helped build an early version of what became Threads (on which I am almost entirely inactive these days). I'm not surprised Bluesky's been more successful than Threads as the X-alternative:

* A lot of the popular content on X is political; Threads' decision to downrank or ban political content makes the posts less engaging. It's understandable from a certain perspective — a lot of the political content is just dishonest ragebait (and I learned to unfollow most political X accounts as a result). However, one of the most interesting things about X is that it's where news travels fastest, and it's significantly less-filtered than traditional journalist editorialization. It's hard to compete with X if you don't allow it, or downrank it.

* Obviously Elon's massive political spending will have an impact on the userbase of the product, since the spending is so one-sided. X has become dramatically more right-wing since he took over, and even more so after Trump's campaign, so there was pent-up demand for something else... Especially something else that allows left-wing political content (i.e. not Threads). There are a lot of conspiracy theories about Elon changing the X ranking algorithm to favor right-wing content, but TBH I think it's simpler than that: more left-wing people left the site (or used it less) due to distaste for Elon, and Elon unbanned hordes of right-wing accounts, who are very enthusiastic about supporting it.

* There's also one way in which X is (openly) biased in its ranking: paid accounts have their posts ranked higher than free accounts... And paying for X is public: you have a blue check next to your name if you pay. In right-wing spaces, having a paid bluecheck account is a badge of honor, in part due to it supporting Elon; in left-wing spaces, it's a badge of shame for the same reason. Although from a technical standpoint the ranking change isn't specifically political, in practice it's very political: many more right-wingers are bluechecks, so in practice you see more of their content and less left-wing content. Bluesky doesn't have paid accounts, and even if it did, it wouldn't organically result in right-wing content being prioritized to the same extent, so for left-wing users it's a much friendlier place.

* This is perhaps more of a personal gripe, but Elon Musk's decision to downrank external links has been pretty bad for the quality of X content. I understand the reasoning: he's trying to keep people engaged in-app, and external links make you leave. But plenty of interesting people use X as a promotional channel for their (interesting) content, which I want to see, and which often doesn't suit tweet formats. So by downranking posts with external links, I see less of the things I actually am interested in. I don't think I'm alone in this, and Bluesky doesn't do it — and I've heard (anecdotal) reports of people really enjoying being able to more reliably engage with that kind of stuff again on Bluesky.

Mastodon is too complicated and segmented to take off. But Bluesky isn't, so I can see why it's having a zeitgeist post-Trump's-election, which Elon helped fund. I'm not sure how much Bluesky will manage to retain its current spike in users: Threads at one point had a similar spike, and most users churned back to X. But it makes sense to me that Bluesky is a more natural home for the ex-X diaspora than Threads, given the reasons many of them left X.