Interesting question. Not sure if there was a specific reason behind, or if it was revealed why?
Guess it was just a random stage name chosen by the modeling agency at the time. 浜崎くるみ (Hamasaki Kurumi) is definitely more promotion friendly compared to 濱崎歩 (Hamasaki Ayumi; birth name). "Kurumi" is the Japanese word for walnut. It it also a given name for women.