Board Thread:Naruto Discussions/@comment-24837927-20140421121126/@comment-2120712-20140421202521

Because its a jutsu that originates from the dna and can not be learned or taught without having that dna. As for why Wood release has not returned naturally, you will just have to ask Kishi about the recessive and dominate genes that control Hashirama's wood release and how it reacts with other genes down the generations.