Why do some babies walk so much earlier than others?

My son is almost 14 months and apart from a few wobbly steps here and there and cruising like a champion, he shows no real interest in walking at all. He doesn't even like holding my hands and walking. He's rather be crawling. He IS a super fast crawler, though. I know he's still in the normal age for walking and that he's fine, but I am curious as to why some babies walk to much earlier than others. I have a friend whose baby is 6 weeks older than mine and her son was walking before 9 months old. My nephew was walking by 10 1/2 months old, but his sister was walking around 14 months old. I'm getting tired of the "Is he walking yet?" line of questions by well meaning acquaintances, usually followed by some condescending "advice". I am trying to be patient with him and let him grow and develop at his own pace, but sometimes it feels like he's never going to walk. He's been cruising and crawling since around 8 months old.