Debian stable releases don't come with the latest stable releases by the upstream developers. The upstream developers wouldn't have released their software if they didn't think those were stable. So why on earth Debian doesn't comply? What are the criterion for software to get in the Debian stable repo?
The upstream developers wouldn't have released their software if they didn't think those were stable.
It's not because it works perfectly on developper computer / tests cases... that it will work on all other cases.
Finding bugs on software is slow and the more testers you have, the more bugs you will find.
What are the criterion for software to get in the Debian stable repo?
To be included in "stable" release, software needs to be tested in "unstable" release ("sid").
Once the software has been tested long enough in sid, it can goes to "testing" release.
When debian team wants to contruct a new "stable" release, the freeze period begins: (see: https://release.debian.org/jessie/freeze_policy.html)
When the "testing" release has been tested enough, it can becomes the new "stable" release.
The "testing" and "unstable" release encounter bugs, you can browse them at: https://bugs.debian.org/release-critical/