Email became popular and commonplace for "the public" when Hotmail and Gmail came along. Before that, you had ISPs offering some POP3 services which people used

Email became popular and commonplace for "the public" when Hotmail and Gmail came along. Before that, you had ISPs offering some POP3 services which people used to send and receive a few emails per week. Maybe your workplace provided email services. But the main thing is; people only used email if someone was willing to manage it for them. And they almost always went with the default choice (ISP or work email), later replaced by the most convenient option (Hotmail and Gmail). There's still lots of people using their 20 year old ISP email addresses because they can't be bothered or don't know how to switch. There is no default or obvious mastodon option. That's the problem.