Under international rules, he couldn't wear his college/pro number 23, because numbers above 15 were not permitted. As Wiki puts it:
"Up to 2014, players in FIBA-organized competitions for national teams, including the Olympic Games, World Cup and Women's World Championship, had to wear numbers from 4 to 15."
SO that explains why not #23. But why 9? Because Michael's older brother wore #45 when they played together in high school, and 4+5=9. In fact, that's when Michael picked #23, because it was approximately half of 45.
"Jordan explained in his autobiography that the 45 also played a role in his Olympic number nine. Jersey numbers on Olympic basketball teams only go up to 15, so Jordan came to the number nine by adding four and five together."
By the way, just as a throw-in for those of you who wonder why you never heard of Michaal Jordan's one-year-older brother Larry, he's only 5'8" tall. When Michael famously failed to make his high school team as a sophomore, he was around Larry's height -- and Larry was the star of the team. Then Michael had a significant growth spurt, ending up at 6'6", which made all the difference in his basketball future compared to Larry's.