Why was the United Nations more powerful than the League of Nations?
There are a number of reasons.
Firstly the USA was a member of the UN whereas it was not a member of the League. This was even more significant in 1945 with the emergence of the superpowers and in particular the global role of the USA.
Secondly the UN had the power to take much stronger action to prevent conflict. The failure of the League was highlighted in the Japanese invasion of Manchuria, the Italian invasion of Abyssinia and the German re-occupation of the Rhineland, none of which were prevented by the League.
The UN had the power of putting in peacekeeping forces to try and prevent conflict or an escalation of conflict. However it would be wrong to assume that such actions have always been successful. They have not, and the limited mandate of such operations has repeatedly failed to prevent conflict since 1945.