I think the wide adoption of whatsapp also has helped slack. Because the way it works is similar. (almost) everybody loves whatsApp. It also has the addiction (do not miss out!!) part and ease of sharing options. Slack has extra the wide integrations + use platforms (mobile, desktop).
I just started using slack in the company I work for. Messages are similar to whatsApp (will be late, hey look at this) Strange that whatsapp did not take/get a share of this type of business usage.
What will happen if whatsapp implements the slack business model (pay to view history)?