The reason for this is the power that permanent membership of it yields through the all-important possession of the right to veto.
The structure of the UN is one that, it can be argued, proved to be successful. Logically, one can say that the structure is efficient; every member is represented and has the opportunity to air its views and grievances via the General Assembly, and for the purpose of the vital issue of maintaining global security, a Council of powerful and generally cooperative states exist.
The delegation of different tasks to the General Assembly and Security Council gives the opportunity for clarity of purposes, and for the clear setting out of who is responsible for exactly what. For instance, the Security Council is fully aware of its personal responsibility of maintaining peace and security. This is a major advancement from the core structure of the UN's predecessor, the League of Nations, where "there had been no clear division of responsibilities between the main executive committee and the League Assembly" (Taylor). One can note how such a distinguishment could, both in theory and even in practice, lead to a more anticipatory and reactory surveillance of conflict, as it reduces the bureaucratic time-wasting that would be entailed if all countries were to be collectively responsible. As Taylor says, "The elite membership of the Security Council yields benefits in terms of speed and cohesion over the slower and more divisible debates of larger bodies".
A Realist may look at the current structure and declare it a beneficial one to realising the UN's aim of achieving international peace and security. This is because the UN's structure, intrinsically, is one of power. Five members, all with substantial international leverage, are afforded the responsibility of maintaining global order, and are also given the power of the veto. A realist would say that this is rational since, after all, the world is governed by power.