Maturity, independence, grown up years (25 - present +).
I am asked this question quite often, and even though the true answer is complex, the shortest version I could give in most situation is: because of a broken heart. I first came to spend 5 months on an old sailing boat in an Oregonian marina.
I know, it sounds strange, and there is more to it than that.
My reason for not having a child is complex, as reasons usually are, but it can be summarize like this: I did not find a possible loving father for my child with whom I could create the required stable and resourceful environment.
Being unwanted child myself, I was never willing to impose such or similar situation on another being to satisfy my instinctual needs. I strongly believe that children should be born into love and excellence, and require two deserving parents who are eager to connect with the young human beings emotionally.