Whilst i believe this article on the UN's loss of authority on peace is unfair to Guterres(whose term started at the same time as trumpism polluted affairs of world and citizenry) its point on why no new multilaterals especially with fall of berlin wall is on-song

after the end of the cold war the United States, finding itself in the position of hyperpower, did not want to be trammelled by any unnecessary global rules. No new regional—much less global—organisations were created, save for the rather inconsequential European Bank for Reconstruction and Development. This is in contrast to what happened after the first and second world wars, as with the foundation of the League of Nations and UN respectively.

What is more, UN rules were openly violated. After the end of the cold war, the US and its allies attacked five countries on four continents without UN authorisation: Panama, Serbia, Afghanistan, Iraq (the second war) and Libya. (For Libya, there was a UN Security Council resolution but its mandate of civilian protection was exceeded by the overthrow of the regime.) France and the United Kingdom, also veto-wielding members of the Security Council, participated in most of these violations of the UN Charter, even if France refused to go to war against Iraq. And Russia attacked Georgia and Ukraine (the latter twice).

Thus these four permanent members broke the charter eight times. Among the permanent members, only China did not do so. The UN as a collective-security organisation, whose primary duty is to protect the territorial integrity of its members, has failed in that role—simply by being ignored by the most powerful states.