That puts the onus on every single person to maintain the history of the project. Lots of people delete mail (not me, but lots of people). Also, you can't retain history of what was sent before you joined. Also, emailing individual documents means people can't research by themselves.
I mean, email isn't meant to be a document repository. Slack (or other chat) is great for immediate conversations with many people. Email is great for slower conversations with a more limited number of people. A wiki is great for a very slow conversation. An actual document repository is great to be able to link to when using any of the other conversation methods.
Just because Slack isn't good at slow conversations doesn't mean it's bad. Just because email is bad at including people who joined after the fact doesn't mean it's bad. Those aren't their strengths, and that's fine. They still serve their own purpose just fine.
