Why doesn't the latest Ubuntu edition come with the latest stable Linux kernel version?

Solution 1:

Like Ubuntu, the Linux kernel also has long-term support versions. So, for an Ubuntu LTS release (like 16.04), picking the latest long-term kernel version makes sense. This way, unlike with a normal stable release, Canonical can rely on the kernel maintainers to backport fixes for a long time. 4.4 will be supported till February 2018.

Canonical did pick a normal stable version, 3.13, for Ubuntu 14.04. This was considered a mistake. The Ubuntu community and Canonical devs had to pick up maintenance of the 3.13 kernel the same month that 14.04 released. On the other hand, 3.12 would have been supported by the kernel team until next year.