Why don't the Ubuntu repositories have the latest versions of software?
An Ubuntu release goes through several stages before it actually makes it to the public as a finished product:
Some time before Ubuntu launches a release it freezes its packages at a certain point.
Before a release is out but after the package freezing, work is done mostly to fix all the bugs and issues that there might be in those packages. New package versions are not imported into the repositories anymore after package or feature freezing.
Once the release happens additional changes to those packages only happen for bug fixing and security issues. There are no more upgrades done to the packages in the official repository even if new versions of the packages are released.
New version of packages are consistently being imported (from Debian) for the next release of Ubuntu, until the next freeze happens and the same process repeats itself.
As an example, you can have a look at the release schedule of 12.04.
You can see that even though 12.04 was released in April, in January 12 something called Debian Import Freeze happened.
This is just the first of many freeze stages happening before the actual release and means that at that point the import of packages from Debian testing or unstable stops and work starts on them to customize and fix issues with them.
No upgrades are done after that point in a lot of packages and the version that package had at that point is the version present and maintained through a release's lifetime.
So even though there are higher versions of the same package in developers' PPAs or in Ubuntu+1 repositories those will only be included in the next release of Ubuntu.
This is done for stability, security and functionality. New bleeding packages being imported all the time to the main repository would mean issues and a lot more problems to be solved. A freeze in the packages version helps to sort that out and make Ubuntu safer and more stable for the end user.
A new version of Ubuntu is released every 6 months, so every 6 months new packages are prepared, tested, customized and released with a new version. Future versions of a packages can be installed in your system via a PPA or just by downloading it from a web site, but the version of the package in the official repository remains the same.
For more understanding and an interesting overview of what happened to Ubuntu from 10.04 until the launch of 12.04 have a look at ReleaseSchedule - LTS to LTS and Stable Release Updates page for a complete overview and explanation of an Ubuntu stable release.
Two reasons. The first is quite obvious: it requires a human to spend time updating the package when a new upstream comes out. The second is that if you are running a stable release as opposed to the current development version, packages are intentionally NOT updated willly-nilly to avoid breakage. See http://wiki.ubuntu.com/StableReleaseUpdates.
Packages are frozen for the release and not updated subsequently for a number of reasons. If new releases were brought in post-release, then the new version...
- might bring new bugs, thereby regressing functionality that was present at the time of the release
- needs manpower to package, test, and upload
- needs its own set of security updates
- would need updated translations for its UI
- would need updated documentation (and translations)
- makes technical support more challenging
- might annoy users who have gotten used to the features in the old version
- may require newer dependencies which could break other apps if they were changed in the repository
- may break other packages that depend on this one
- may break user scripts, templates, tools, etc. created for the old version
All that said, be aware that there are cases where Ubuntu does do full updates of software versions in the repository. Firefox for instance.
Also, there is a ubuntu-backports repository users can opt into which updates software packages that won't cause problems like those listed above. It's not enabled by default so users have to opt-in to it, which is done to eliminate the surprise of having your software change out from under you. Also, it's not heavily staffed and so I'm not sure how frequently packages actually get updates.
Further, the SRU team has recently updated polices a bit which hopefully will make it a bit more straightforward to get bugfix-only package updates in.