If the UN was established after WW2 with Allies, one of which was Soviet Union, establishing it, how comes soviets agreed to headquarter such an important organization on not a neutral ground i.e. Switzerland? Why no major branch of that organization was in Soviet Union (i.e. International Criminal Court in Netherlands, World Food program in Rome) etc..
I understand that US may have been paying and is still paying a large part of its expenses but from soviets' perspective they should have not allowed it..
There were several overlapping factors that favored the United States as the location for the permanent headquarters of the UN. Firstly, the vast destruction the war had unleashed precluded many of the potential European choices. Even relatively undamaged European cities were often surrounded by devastated areas. Neutral Switzerland was floated as a possibility, but there were a number of reasons that mitigated against this choice. The Swiss put so many preconditions upon using their territory that it was an unattractive option. The perception that the Swiss-based League of Nations had moved from one failure to another cast a long shadow. For this reason many of the non-European UN members argued for a location in the Western hemisphere. The Mexican delegate to the UN Preparatory Commission, Luis Padilla Nervo, stated that the new organization
ought to have the advantages of a fresh start in an atmosphere of faith and optimism, freedom and progress. And we believe that at the present moment most of these conditions do prevail in the American continent.
The Chinese delegate, Victor Chia-tsai Hoo was more forceful in condemning a European location, claiming that " 'the spirit of Geneva' became almost similiar to 'the spirit of Munich.' "
The attitude of the two superpowers would prove vital for the eventual decision to locate the headquarters in the US. Publicly, the Soviets backed an American location; the Soviet ambassador to the US Andrei Gromyko opined in late 1945 that:
The United States is located conveniently between Asia and Europe. The Old World has had it once, and it is time for the New World to have it.
Gromyko's argument against a European location, especially Geneva, was likely because such a location would be an uncomfortable reminder of the USSR's rocky relationship with the League of Nations. Although he did directly indict the League of Nations, Gromyko did say that the position of the USSR on the League was in concurrence with its Chinese counterparts.
For their part, the US stumbled its way into an American location for the headquarters. FDR initially favored an neutral island location like the Azores or Hawaii, but these was impractical. FDR tasked US Secretary of State Edward Stettinius, Jr. in early 1945 to find a suitable location. Stettinius favored San Francisco because it would help marshal UN forces for the war in Asia and the boundless Pacific Ocean would be a symbol of the UN's hopeful new mission. Other cities like Philadelphia, Chicago, Detroit and even the Black Hills in South Dakota also angled to be chosen for the site of the new "capital of the world." New York City rose to the top of the pile in no small measure because John D. Rockefeller Jr.'s gift of $8.5 million to buy land on the East River to pay for the UN headquarters at the end of 1946. The timing and generosity of Rockefeller's "gift" (he made the offer after getting a guarantee it would be free from federal taxation) coupled with the allure of a skyscraper in the heart of one of the most modern cities of the world was too tempting for the UN planing committee and they quickly resolved to chose New York City as the headquarters.
Sources
Mazower, Mark. Governing the World: The History of an Idea, 1815 to the Present. New York: Penguin, 2013.
Mires, Charlene. Capital of the World: The Race to Host the United Nations. New York: New York University Press, 2015.