Why did so many nations gain independence after World War 1?

Because lots of countries were under the superpowers' rule. For example, Ukraine, the Baltic countries, Finland and etc were part of the Russian Empire, Croatia, Romania and Bosnia were under Austria Hungary and etc. The war destroyed Europe, so they had to rebuild themselves afterwards, and could not afford to maintain troops(they didn't even have enough troops, as so many died... not sure about this though) to control the other nations, others were Empires that crumbled and divided into smallers states(i.e. Ottoman empire). So they took the chance and fought for freedom, as their former rulers were in no position enter another war. Others were created by the Allied superpowers, like Yugoslavia, Czechoslovakia, and Poland just to have influence without ocuppying them and also to reduce Germany, Austria and Turkey's power.