Google announced the new version of Android 7 is called Nougat on 22 August 2016. It features per file encryption, multi windows, better Doze and better VR performance.
Nougat has a new API for graphics called Vulkan. This is not supported by some popular SnapDragon SOCs, eg 801 which powers many popular phones. Without this support, the build will not pass Google CTS and hence no official build for Nougat for Nexus 5 and OnePlus One.