The Ubuntu development cycle is not in sync with the Linux kernel, so the latest version released is not usually implemented. As it stands right now, the Linux kernel used in Ubuntu 15.10 is 4.2, and it’s possible that a new one will be made available soon after launch, maybe right before.
When a project hits one of these freeze milestones, whether it’s for software stacks or the kernel, no more upgrades are being made. This allows developers to properly test everything and make sure that everything is in order for the launch.
Ubuntu usually ships with a specific branch of the kernel and then sticks with it. Even if a new version of the kernel is released, they don’t usually upgrade it, at least not for the non-LTS versions of Ubuntu which are only supported for nine months. On the other hand, the Ubuntu LTS versions do get kernel upgrades with each new point release.
Submitted by: Arnfried Walbrecht