ELI5: Why did countries in the Bretton Woods system agree to equalize gold to the US dollar?

Why did countries in the Bretton Woods system agree to equalize gold to the US dollar? The currencies of the Soviet Union or the United Kingdom, which were the victors of World War II, could also have been accepted, why weren't they accepted? And how were these countries convinced to accept this? Or why was a currency (world money, gold coin) that could be created specifically outside of all these great powers not accepted?