[jum1p]

SMTP is simpler. XMPP is extensible.

SMTP was first so one registered accounts with email and not XMPP. Companies send stuff like bills via email.

There were various popular providers -- today Gmail is the most popular one but others like AOL were popular before. It's hard to change one's address.

Sometimes, your ISP gives you an email address.

Email is formal, so job application happens also over email (at least in not very technical jobs).

Probably because email was so important, there were and are still more people who host themselves, including companies (companies with their own xmpp setup are rare).

The rest of email is today (mostly) at Facebook, Twitter or LinkedIn.