It seems to me that Mastodon gained traction in a way that wasn't much different from email. In both cases there was a communication network which anyone could add a node to pretty easily. Your node could have local moderation policies and you wouldn't have too many issues with getting blocked as long as you acted nice. For both email and Mastodon this enabled a few motivated administrators with some influence to onboard thousands of users quickly, and benefit from a network effect bigger than just their node.
The similarities to email end when you consider _why_ people migrated to the network in question. With Mastodon the waves have been culturally/politically motivated people running away from something (ex-Twitter users fleeing abuse, Japanese loli enthusiasts trying to avoid embarrassment, sex workers seeking an alternative to SESTA/FOSTA regulatory deplatforming). Whereas with email people were running _to_ something - the first free, global communications network of its kind.
It's also interesting to note that email nowadays has lost these advantages (running a server is much tougher now and you can't really pull it off without blessings from Google and others). Sure enough email is now on the decline.