On the surface, I would say that the reason is because of my health - physical and mental. I never feel well physically, and this affects my mental state, in addition to the depression that runs in my family. It may seem selfish, but in order to manage the depression, it requires a lot of self-attention and rest, which works wonders for me. Stresses and curveballs completely throw me out of whack and turn me into an abominable drag. Choosing to be stressed out all of the time is not a good choice for me. Being a parent, as I have heard many, many times from parents, is stressful and full of curveballs.
People have tried to tell me (and I have considered myself) that having a child will give me purpose and put my mind on another human being instead of myself, implying that my childfree existence is self-centered and unhealthy, and that my mental and physical health is due to this selfishness. But, I saw my mother (who has major depression) suffer horribly with us kids to the point of becoming suicidal, so I know that's not true. I think it's a sound and respectable decision to look at the big picture and realize that what's good for one person may not be good for another - for their life or for the world - despite the fact that friends will tell me that my husband and I would make great parents. Personally, I think the reason we would make great parents is the very reason why we're not parents.
That being said, if I really WANTED a child, I would make it work somehow. Ultimately, the real reason is that I LOVE my freedom.
