Answer:
Because they wanted to create an institution whose task will be to prevent conflicts in the world. That is the reason why League of Nations was created after WW1.
Step-by-step explanation:
After WW1 ended Allies, especially American president Wilson believed that some body should be created to prevent this type of wars to happen again. League of Nations was created, but it didn't reached its goal. After WW2 similar idea emerged. This time, mistakes that were made when League of Nations was created were not repeated. All great powers decided to support the creation of UN.