The story there is more complicated, and security plays a large part of it.
1) Slack uses AWS. While it's perfectly possible to operate without slack, chat services are valuable during outages. The kind of incident where it would be most valuable is precisely the kind of incident that would take it down. Communication services must be completely independent of the AWS infrastructure. Likewise, something like PagerDuty is effectively off limits. Though they've done a good job building resilience, PagerDuty have been affected by AWS outages.
2) Slack communication isn't encrypted end to end. While it may be stored encrypted at rest, it is stored in a method they can decrypt, read and analyse (inherent in the features they provide). Chat is filled with critical operational information, and (no matter how well you train people) passwords and other critically sensitive information. There's always an extremely slim chance that a rogue actor could choose to access information they shouldn't. That information is, also, potentially accessible to someone who might hack in and compromise the service. We've already seen Atlassian's hosted HipChat service compromised and some chats leaked. Slack has, arguably, an even bigger bullseye painted on it. (fun thought exercise: what damage could a malicious actor with access to yours slack channel do with your chat ops bots? How long would it take to stop them?)
3) feeding on from 2, to some degree, you don't control the security of hosted solutions. Audit compliance is a checklist game, it is perfectly possible to check every point on those checklists and still be inherently insecure. If you're even remotely paranoid, you've got to assume that anything you don't host will be compromised, and weigh up the risks associated, against the value and necessity of the service. Leaking operational information is a big, big risk. Amazon's security sets a high bar for paranoia. They are determined to never have a New York Times front page article "Amazon hack, credit card information stolen", or similar.
There's just no strong value in them switching to Slack, just a whole mass of unnecessary risks.