A lot of reasons:
1. Emails and attachments go stale. Stories can be updated in real-time without any involvement from your recipient
2. They’re not easily share- able. Giving the right context in long mail threads, attachments going missing because of a reply instead of a forward end up creating a mess.
3. They don’t give you instant, deep insights. When you send a Story, it's the one place you can use as a central destination for your materials. You know who opens it, when, and for how long, making it simple to figure out who’s really paying attention.
4. They’re not secure. Emails don’t have any restrictions on forwarding. Stories can be locked down to specific emails or domains whitelists or blacklists, with passcodes, or email gates.