The whole point of the UN is to avoid a major war between nuclear states. It is not a reward for countries that happen to share our values. The UN was designed to permanently cement the post-WW2 powers as the major powers, and to provide a forum for them to resolve major differences rather than feel like they have to resort to lobbing atom bombs at one another. Everything else the UN does - from the participation of small states to the very important humanitarian initiatives - is of secondary concern.