Touting "industry standards" and saying you take security "seriously" is one thing, but building a product from the ground up with no purposely designed backdoors is another entirely.
Slack, and most other companies relying on in-transit and/or at-rest encryption, have a security posture that is extremely similar to email - which, as high profile hacks over the last few years have shown us, is not very secure.
Additionally, software applications such as Slack, Dropbox and Teams have suffered numerous significant security breaches; for individuals or companies concerned with privacy and security, this may be unacceptable.