This is all volunteer based, so if a device isn't supported, it's because no one stepped up to maintain it

The OnePlus 2 was pretty unpopular because it didn't have an NFC chip, so there's probably no one who owns one who is willing to maintain it.

The OnePlus 2 was a bad phone overall. It didn't sell as well as the OnePlus 1, had the infamous Snapdragon 810 and the modding community was never that strong. All this ends up affecting long term support.