You didn't miss it, I didn't go into detail on it in the article ... one big reason is that because of how ActivityPub works you only see a fragment of the overall conversation (instead of everything). If you're on a big well-connected instance like mastodon.social you see more of it but still not all; if you're on a smaller not-so-well-connected instance you miss most of it. This comes in conversations (the "missing replies" problem), with search, and with hashtags.

Another reason is that Twitter's got a lot of journalists, activists and organizers, politicians, government agencies, athletes, etc ... and Mastodon for the most part doesn't. That's not a technical issue, but for most people, following one or more of those groups is something they're used to from Twitter, so Mastodon doesn't fill the same role.

Again, there's plenty of stuff Mastodon is good at! And Twitter clones replicate Twitter's problems as well as what people like about it. But for people who are sick of Twitter and want a similar experience elsewhere (as opposed to trying something different), they're more likely to get what they want on Bluesky (and in many cases even Threads, especially if they already have an Instagram account and don't want to see political stuff) than Mastodon.