Why did Slack sell to Salesforce?

The simple answer is because Salesforce was willing to pay a ridiculous multiplier - something like 70 times revenue.

Slack came dangerously close to making Dropbox’s mistake of thinking it’s a platform. Sure, you build stuff for Slack, but the API surface for them is tiny. They’re extremely easy to port to other chat platforms.

With Salesforce, they can leverage an experienced - well, sales force - to push Slack, but more importantly, to truly make it a platform for users to more easily interface with Salesforce’s other products.