UN for human rights to oppressed people
It would be ideal if the UN had such power, but that would mean the members states must agree.
The UN was created to avoid conflicts between nations and it's a lot easier to convince governments that wars are to be avoided at all costs than to have them accept their own policies are so bad they necessitate a military intervention from an international institution to suppress them.
What happened recently with a NGO in Chad (displacing children from Chad thinking they were from Darfour) is no good argument in that favor.