23 September 1931
1945
Original Answer: Because they are decades away. Improved: The United Nations are not a country. They couldn't have "joined" the League of Nations. The League of Nations was the predecessor of the United Nations - the League of Nations failed, and so, many years later in 1942 (I believe it was that year) they founded the United Nations as an improved "League of Nations."
The league of Nations met once a year
2002
1955
1946
1985
1965.
1961
1971
in 1945
1940