Threads is polished in a way that Mastodon isn't. I'm part of a few Japanese subcultures and most of them couldn't make heads or tails of the Fediverse despite the existence of Misskey. Threads though? They signed up through Instagram and were posting in minutes. I really want Meta to commit to federation, even if existing Mastodon communities defederate so that techie weirdos like us can live their dreams of running custom clients and interfacing with regular folks (like XMPP federation with GChat back in the day.)
Yeah, Twitter is super big in Japan for mainstream teen and young adult users in the 18-25 city student/young professional demographic. Same with Instagram. Mastodon adjacent stuff like Misskey, Pawoo, and Mstdn got the reputation for being for hosting weird, niche, or pretty much illegal content, and nobody really understands the point of Fediverse.
Also, mastodon just isn't easy like twitter/reddit/insta is. There's hurdles you have to get past. It's to the degree that, despite being the target demo for masto, I still haven't really gotten into it. Too many hoops. Even a flippin' bbs seems to make more sense at this point
A friend of mine tried mastodon on my recommendation. But he couldn't find a "login with facebook" button, which is how he assesses whether a website is trustworthy. Mastodon to him is "like a scam version of threads". You'd be surprised how prevalent this view is with normal non-hn crowd.
A good chunk of them did, and some of them stuck around. I've had an account going back to 2017, and my feed is far more active today than it was before Elon purchased the site.
Honestly its amazing that as many people went with mastodon as they did.
Ideologically motivated (i.e. "federated-first") open source projects never succeed in attracting mainstream users. They have to make too many compromises, and inevitably prioritize their motivation (federation) over what is necessary to attract the mainstream (e.g. excellent UX).
The fact that mastadon is doing as well as it is, is sign number 1 that the market is begging for something like threads to happen.
On Twitter and other centralized social media this is very easy to do. On Mastodon, it is not. I have to copy the URL, paste it into a text field on my instance, and then like it.