I am bit shocked to see that Germany joined United Nations - UN in 1973, what was political reason for postponing membership in that important organisation, if we see that for example Italy joined UN in 1955 and Japan joined UN in 1956?
The reason was very simple. There were two Germanies. Soviet Union would veto the Federal Republic joining. (From the Soviet point of view it was illegitimate). For exactly the same reasons US, England and France would veto the German Democratic republic.
They could only join when they (and all others) recognized each other as independent states. (Moscow treaty, 1970).
Somewhat similar situation existed with PRC and Republic of China (Taiwan) for some time. (Until 1971 RC represented the whole China in UN, but now it is PRC, and RC is not a member).
In 1969 West Germany switched from Hallstein Doctrine ("exclusivity" -similar to One-China policy) to Neue Ostpolitik ("detente" - normalization with East Germany et al) which led to UN membership by removing objection from the USSR (which had veto power).
In addition to the comments by sds and Mark, let me note that at the time West Germany was extremely reluctant to do anything which would give the appearance of accepting or legitimizing the division of Germany into the FRG and GDR. The price of admitting the FRG to the UN would likely have been to admit the GDR as a separate nation. West Germany was not willing to pay that price.