The last part I think happens because the people who are there are most likely the ones that dislike very short content, which is what Mastodon offers by default (I know it depends on instance, some allow 1k or 5k character toots), but it's a disorganized mess. It's a kind of service/software for fast communication that can be done better in a specific Discord server. The "good stuff" will always end up posted elsewhere, like personal blogs or websites, where it's better organized (most of the time)

One thing that keeps people on xitter is the "it's where I get the news". Major news outlets aren't on Mastodon and likely won't be. There's also the stuff that "you get to know before it becomes news", which also won't be on Mastodon because it lacks the "gossipers" and the mass of users that is needed for having people "everywhere"