Because email is threaded conversations, while slack is not. I can write 5 different emails and respond to each thread individually, but I can't do that in slack: I can write 5 messages, but my peer(s) can't answer each one individually - they can just write 5 messages and I have to figure out what they respond to.
Email has the added benefit of working nicely when you're on a connection that's not always available (mobile, traveling, etc.) I can write the message and have it sent later, download all messages and read (and answer) them without a connection. Slack (and other chat applications) just falls flat on it's face when the connection drops from time to time.
Now, all of that might be properties that you don't need because you're never traveling and always on the same schedule, so that you're able to answer questions in real time - but don't assume that's true for everyone.
What I can tell you is that it factually has wiped out internal email for our team at Charge.
And I think that's pretty common for most teams that have used it. Why write a slow clunky old email when a Slack message will do?