We haven't been able to use any third party tools because our security people don't feel great about giving third parties write access to everything (including our source) for tools that don't need it. In the past, GitHub hasn't differentiated write access to issues (which many tools need) and write access to the source itself (which basically nothing should need).
Considering all of the PCI-DSS / HIPAA / SOX / etc. audit points around change control (even ignoring corporate versions of the same) it's practically impossible to add external services to GH and have them be useful while still meeting compliance. Change control is required, but that also implies control of change control. As-is any service could delete all of your content, whether intentionally or inadvertently, or even worse could corrupt or otherwise alter your history.
It may be detectable and recoverable because git, but it would be infinitely easier to have code and PR be r--rw-.
EDIT: This is now a thing, according to @bhuga below. Also, GH is now publishing a roadmap for what's coming down the pipe, so we're not in the dark as to when these critically-important-why-isnt-it-out-yet features are being worked on. https://developer.github.com/early-access/platform-roadmap/