It usually takes distro developers a while to vet kernels and ensure stability with their userspace stuff. 3.7 and 3.8 had a few regressions, and were skipped by a few distros. 3.9 only made it into Debian Stable last month (as a backport from testing).

On Ubuntu, you can always get some newer kernel versions through apt-get, but the default will always be what they consider to be stable (especially for an LTS)