The absolutely most important reason: everyone has an email address. It’s not just the least common denominator, it’s pretty much the only common denominator. RSS practically does not exist outside the tech scene.
The main paradigm in RSS, also, is to make it look like e-mail.
Every feed reading mechanism I've ever tried that was usable looked like a mail client. Feeds look like mail folders, and arriving items look like mail delivery, with UI notifications like "Foo (15)" indicating that feed Foo has 15 unread items.
The advantage over e-mail is that it is pull; you don't have an RSS identity that can be spammed. Kill a feed and it is gone.
Everyone having an email address is a false assumption, or a bad generalization. There are people among us without one, and owning a phone number instead.
I'm not sure along which lines the divide falls though. Younger? Less western? Less businessy? It's still possible that the people who would be interested in a newsletter are the same people who have email addresses.
Every feed reading mechanism I've ever tried that was usable looked like a mail client. Feeds look like mail folders, and arriving items look like mail delivery, with UI notifications like "Foo (15)" indicating that feed Foo has 15 unread items.
The advantage over e-mail is that it is pull; you don't have an RSS identity that can be spammed. Kill a feed and it is gone.