Mastodon is made with the ActivityPub protocol and Mastodon is a notable social network implementation of the ActivityPub protocol so when everyone is talking about the decentralized social network that they are joining after leaving Twitter, they will say Mastodon. If someone is talking about a federated decentralized protocol, then you will hear them say ActivityPub.

Your comparison is like asking why people say Wordpress when they mean CMS platform. I joined Mastodon rather than I joined the ActivityPub protocol. I joined Wordpress rather than I joined a CMS platform.

The "Chrome/Web vs Mastodon/Fediverse" isn't event correct, it should be "Chrome/Web vs Tusky/(Mastodon|Fediverse)", or "Twitter/Web vs Mastodon/Fediverse"?. Those things operate on a different level, they can't really be compared because they serve a different purpose.

Twitter users aren't joining the Fediverse, they're joining Mastodon. They rightly don't care about the Fediverse, they're just looking for a Twitter alternative, that's what Mastodon is. The Fediverse isn't tangible like Mastodon is. For the majority of users, that's an implementation detail.

Because it's already complicated enough. Marketing matters, even for open-source projects. If you want people on your decentralized social network, don't make it sound more complex than it already is.

Because ActivityPub is just the protocol, and they mean the platform and social network family.

So, more like saying Blink or Chrome instead of http.

I've never heard about ActivityPub before your post. And Mastodont is complicated by it self. Is it one server, one service or multiple ones? Why handle isn't something simple. Where are my messages stored? So put on top of that protocol and it would be a total mess.