Chris DiBona has spoken in public about Google's reasons, but he was kind. I cannot discuss other experience I have publicly, so I'd simply suggest doing some research on the large unknowns in deployment of AGPL software underneath a proprietary Web service somewhere, and then think about all the integrations you'd wire up to something like a Slack derivative.
I am not a lawyer and would strongly suggest consulting one before using AGPL software anywhere in your infrastructure, at all.