The "federation" of e-mail and Mastodon are only superficially similar. It's non-intuitive to follow a user from a different instance. It's also non-intuitive to find those users to follow. Most of the time Mastodon users are found via some side channel where they advertise their presence.

Every point of friction you add for a user of a new thing is a reason for them to abandon it. Even if Mastodon isn't trying to necessarily replace Twitter, catering only to the most dedicated users is not a way to grow a healthy community.