The fediverse has conversation streams that are mostly in public indexes, which other people can observe and interact with. In addition to these conversations comprised of statuses representing activities, it is also possible to follow the authors providing said content so that their latest posts show up in your stream, regardless of whether those posts pertained to that conversation or not.

Email, conversely, often is a limited-scope conversation that can only be observed or interacted with between the people participating in said message. Of course, this changes slightly with the use-case of mailing lists, which often provide a public archive of prior messages. But email content is generally not accessible from the web in the same manner that a status is.