Board Thread:General Discussion/@comment-113.162.173.64-20160221165412/@comment-1504072-20160221172906

No explanation is really given. From a writing perspective, it's because Snake needed to have some Japanese DNA in him. I'm not sure why this reason was chosen over having Big Boss (or EVA) be of Japanese descent though. Storywise, it's possible the surrogate mother may only have been chosen after viable embryos had been produced.