Possibly discrimination--not very likely, but remember that this was back in the 40s. Up until then, every Nobel Peace Prize winner had been European, with the exception of one Latin American winner.
Gandhi was actually nominated five times, the last time being right before his death. In 1948, instead of awarding the prize to him, no one was awarded the prize; they said there was "no proper living candidate", which is basically a nonverbal way of saying Gandhi won. [Why they didn't just give the prize to him posthumously, I don't know.]
It's one of the more controversial Nobel prizes given out, and anyways, it's only an object/title. Gandhi's more revered than most of the other winners anyway.