I disagree. As Mary said, there are a lot of sovereign nations (195 currently). While affluent countries handle disasters "in-house" (Japan, USA, Australia, etc); the ones that need outside support are not likely to want another party to have so much say in planning--until they need it. Heck, we can't even agree as a global population on who needs drinking water.
That being said, multiple organizations allows for redundant supply chains (actually a good thing); plus each organization usually has its own pace and political stance which alters time to ground and access. Countries are responsible for their own disaster response plans--we go to places that don't have adequate resources for the specific disaster, but acquiesce to the local government as needed.
The UN is recognized by members as being in charge; but if they were the only organization in charge, people in Syria would not be receiving treatment right now (as one of many examples of aid organization access and priority).
Only in a worldwide utopia should one organization have that much oversight. In the meantime, multiple organizations equals redundancy, which ensures care reaches affected populations.