Email seems to put my interlocutors in the mindset that results in paragraphs of coherent text that clearly describe an idea. Slack does exactly the opposite.
I receive a lot of one-off Slack messages that border on inscrutable because of lazy spelling/grammar/composition. I then have to engage in a costly conversation to tease out the meaning.
I also receive a lot of Slack messages that border on inscrutable because a 4 or 5 sentence paragraph is delivered one sentence at a time. The messages do not make sense individually, but also do not make sense when read together as a paragraph. People seem incapable of thinking more than one sentence ahead when using Slack. This is particularly bad among the crowd who didn't learn how to use channel-based chat well (e.g., by using IRC).
I get that "hell is other people", but compared to the alternative, email is great. Especially with a good search feature and some basic filters and some auto-replies. And especially compared to modern Slack messaging behaviors.