I've found the opposite - if I start an email discussion, people reply-all from different points in the thread (and some people accidentally hit reply instead of reply-all, and then someone will notice and reply-all just to send that message to everyone. And this all makes it hard to consolidate the conversation.

While if I start a slack discussion, the thread is in once place, and everyone sees the state in real time, so the thread is more coherent and easier to follow.