Xenial is on the 2.1.x branch. Upstream maintains stable branches only for bug fixes. 2.1.5 is a maintenance release and the last version in the 2.1.x branch.
The same happened for Trusty. It is on the 2.0.x branch, and got updated to 2.0.8.1, which is the last version in the stable 2.0.x branch.
Xenial is on the 2.1.x branch. Upstream maintains stable branches only for bug fixes. 2.1.5 is a maintenance release and the last version in the 2.1.x branch.
The same happened for Trusty. It is on the 2.0.x branch, and got updated to 2.0.8.1, which is the last version in the stable 2.0.x branch.