GitHub integrates with any code coverage, code quality, security scanning, or CI solution out there.
Slack similarly integrates with any on-call system, time tracking system, code host...
Sure Jetbrains Space has provided the API, webhooks and SDK, but most teams want to focus on their application itself? It feels a lot "safer" to go with a service that isn't trying to offer everything and then pick and choose. Slack+GitHub+CI of the month+code quality plugin of the month+anything...