Peggy McGuiness at Opinio Juris hits right at the heart of compliance and enforcement of international law. The question is this (to paraphrase): We know why nations sign onto human rights treaties they do not intend to comply with, but why do other nations allow rogue nations to violate the commitments these nations have voluntarily undertaken.
To take this a step further, I think we know a bit about why many nations do not endeavor to enforce international law, but we have not learned how to overcome these obstacles.
In my view, the main problem as to why nations sign onto treaties they do not intend to comply with is the absence of cost. Any political cost in being a member of a treaty you have broken is offset by the political advantage of the publicity received by signing onto the treaty in the first place.
However, this cost calculation is inverted in the enforcement quandary. Enforcement is ENTIRELY about cost. Who will spend the money and manpower necessary to engage in the monitoring, peacekeeping, and risk political prestige in the case of failure. Each nation is advantaged by its peers extending their resources to enforce international law while maintaining their own resources. This free-riding problem is exacerbated by the monopolistic UN scheme which discourages other collective action. The UN doesn't possess money OR manpower, but rather only the willingness of its member states to risk their own. In turn, the willingness of member states to spend their own political and real capital is set upon a political house of cards in the Security Council. As a result, the self-interested willingess of some nations to intervene is precariously perched upon the self-interested power of the Security Council nations. Is it any wonder enforcement is sacrificed?
A serious restructuring of the UN absolutely requires an implicit understanding of the cost obstacles and procedural hurdles to intervention. Clearly a balance must prevail, but there is no doubt that right now the international system is balanced heavily in favor of inertia.