So the way Tumblr works is that your account is basically a blog, with your home page on the site being populated with posts from the accounts that you follow. You can reblog posts onto your own account and comment on them to create individual conversation threads like this one. At one point, there was a bug in the edit post system that let you edit the entirety of a post when you reblogged it, including what other people had said previously, and even the original post. This would only affect your specific reblog of it, of course, but you could edit a post to say something completely different from the original and create a completely unrelated comment chain.
I think this is also true for why people gravitate towards places like Bluesky in more general terms as well. Without even getting into the details of whether or not a platform has an algorithm or whatever other features, whether or not a platform is federated means nothing to the average person and the benefits of the decentralized servers are a disadvantage to onboarding people. When the Reddit exodus happened, I was describing Lemmy to a friend, and when I told him that anybody could spin up their own instance, his response was “why the hell would anybody want to do that.” And this is a guy who ran his own TeamSpeak server for like 20 years.
People don’t want an alternative to Twitter - they want Twitter without the rightwing extremism. Bluesky offers exactly this with an easy and straightforward onboarding process and a familiar UI. There’s even browser extensions to search the people you follow on Twitter and find their Bluesky handles to make the swap easier.
I’ve also seen people praising Bluesky’s algorithm being entirely optional as well as a plus for discoverability. People really like the chronological timeline that doesn’t bury posts - especially artists. I haven’t used Mastodon, and I only used Twitter because all the artists jumped ship after Tumblr banned the porn, but I can say that I have enjoyed how Bluesky works similar to Tumblr in that regard. I’ve never liked algorithmic based feeds, so a chronological feed of the people I follow and the stuff they reblog from other people who I can then go check out as well is exactly the kind of experience I want out of a platform.