Twitter has one killer feature: lots of users. Everything else is just detail.
But they're not, because the majority of Twitter's users haven't moved over. And without the majority moving over, said majority has no reason to.
Federation is basically irrelevant to the success or failure of a platform, since the technical details of how its implemented don't matter to 99% of the userbase. If their friends and idols use Mastodon, they'll learn how to use it. If they don't, they won't.