When it started, Mastodon had an existing userbase to communicate with on OStatus, in the existing GNU Social communities, so it could skip the "Who wants to talk to a ghost town?" era of a social media's growth.

Though this prompts us to wonder why GNU Social took off (modestly) but Diaspora didn't.

A different era. Mastodon had plenty of problems of closed networks to show (before it was Twitter, there was the failings of e.g. tumblr and Google+ to point to)