The UN is a peace keeping organisation. It cannot intervene in countries, just because they are badly managed, where would it start and stop? North Korea, Iran, Iraq, Zimbabwe, Venezuala, etc, etc, etc. It does not have its own army but must rely on the willingness of members to volunteer their troops to undertake the role.
It cannot intervene in warring countries if those involved in the warring are not happy for it to do so. Supposing it had tried to intervene in Syria, despite Assad not agreeing. How would it get there? Where would it land? How would it stop Syria blowing it out of the air or water before it even arrive?. What country would volunteer its army to provide the UN with the forces needed for such a suicidal mission. Would those criticising the UN be happy to see British troops being volunteered for such a mission?
The UN has been working constantly since the Syrian war started, to try to organise a peace deal and broker truces. But you cannot play tennis if your opponent won't let you on the court or permit you a tennis racquet. The UN is often quietly successful in negotiating agreements and truces, but it can only go where it is asked and supported.