Because it doesn't need to improve. It just needs to check some checkboxes. It's "free" as part of Microsoft Office 365. It allows the IT department to check a checkbox and de-incentivize them from looking at Slack. With Slack you have to pay for and is yet another app to administer. Many IT departments who are already Microsoft customers will have an easier time rolling that out than a new app. No new purchase order…
The claim is two years old now and I haven't heard any news regarding the matter and whether Slack proceeded to the end like Epic Vs. Apple or not