I believe that they are so powerless because the UN's entire logic is built on the strict respect for state sovereignty. You do not need me to tell you that one state cannot watch another state interfere in the internal affairs of another state. In the case of the UN, it's exactly the same thing.
In this case, I feel that the situation is a symptom. Burundi is actually coming out of a conflict, and, when resolving that conflict, they established positions that would prevent them from getting into a similar situation again. In order to do so, it defined the sharing of responsibilities in its territory in ethnic terms.
The problem is that the ethnic sharing, as conceived in the Arusha accords, does not automatically correspond to the current reality. Everything has been completely reopened by the current president. In other words, the current president sees that the majority is not Tutsi but Hutu and that power should therefore go to the majority. So he asks people to leave him alone so that he can determine on his own who should govern the country and how. For Burundi, it is a strictly internal situation that is of no concern to the international community and that has absolutely nothing to do with any international agreements. That's what is getting in the way.
The authorities in Burundi and the politicians in charge are wondering where this external pressure is coming from. They do not understand it and the actions of UN committees have no legitimacy, in their opinion. We have been told that. They do not see why they should listen to political opponents instead of the majority in power.
In a word, everyone is completely blind. The president has decided that his government and his institutions are supposed to work in that way. The same goes with reopening the Arusha accords.
Why is he running for a third term when the accords do not allow it? Simply because he wants to run the country as he sees fit without having to worry about pressure from the international community.