The problem with Slack? If you upload your confidential IP, who guarantees the data security? A company would have to trust another company completely. That's not gonna happen with the giants. One little "data leak" and stuff will blow up.
However, there are solutions. Skype for business, just as one example, enables companies to have their own servers. We have Skype for business.
What's the problem with Skype for business? Well, lots and lots of people don't actively use it to its full capabilities and it is still quite limited when it comes to quick group conversations. Save for project features! It's just not Slack :<
As a result, face-to-face meetings and offline projects are still preferred!