I disagree about the MS part. I think Slack would probably do very well there. The enterprise market is where Slack is going, and MS is fully entrenched there. They also have a very developer-centric culture that some people enjoy.
Microsoft would want to buy Slack for the same reason they spent $$$ on best-in-class products like Yammer, Sunrise, rebranding Acompli as Outlook. Slack's revenue is incredible for a startup, but a drop in the bucket for MSFT (for now). More immediately, Slack represents an entry point for Microsoft to upsell Slack's customers to other Office365 services and Azure services.