Why was the nudity not censored in Dragon Ball?
First, in Japan, it was a non issue. Different culture. Check out all the different animes and mangas that pull off similar or even worse moves.

Once it went to the western world where it is an issue... Networks did not realize (sold as kids show -> didn't bother checking the tapes).

Back here there was an issue where, at a kids show on national tv, the host presented the newest show!, ranma 1/2's first episode. Turn out it was the uncensored version, after the episode was over and the camera returned to the host, she was evidently quite upset and embarrased. Myself and my brothers were watching along with our parents, all of us were quite embarrased as well (and most likely everyone that watched that show). It was only then that more attention was put on these shows :)