Why does willpower fail addicts?

I have heard people say that if you want to quit using drugs or stop drinking, you just make your mind up and don't use or drink. I have spent many hours contemplating this. If this were the case, then why do so many people have such a hard time quitting?
I made my mind up when I was sixteen to never use drugs. I was pregnant and I did not want my child to grow up with that lifestyle. There were so many things that came with drinking and using that I wanted my child to avoid experiencing. I found out that those things are not exclusive to the drinking and using lifestyle. However, I went out and drank and used anyway. Was I a weak willed person?
When it came to everything else, I had all the willpower necessary.  stopped cursing. I stopped dressing inappropriately. I stopped hanging around the kind of people who used and drank. I began to eat healthier. I began attending church and following the religious tenants. I seemed to have a strong will power within me.
When it came to drinking or using, the moment it came into my head or someone had it around me, I was like pudding. There was no resistance whatsoever from me when it came to drinking or using drugs. I would become angry with myself and berate myself mercilessly about it, but my pattern would continue with one exception. The time between relapses would shorten. With all the problems and the aweful guilt, how could I contine using and drinking, when I was perfectly capeable of quitting oher things.
Then, recently, I heard someone say, "If you think that an addict just needs willpower to quit using drugs or drinking, you need to think again. Here is a good way to find out how the addict feels when it comes to using and drinking: Go home and take a whole box of ex-lax. Then, use your will power to not have a bowel movement."
This explanation releived all my fears and doubts about my will power. I remembered a time, long before, when my sister and I found my mother's "chocolate" stash in the bathroom mirror. We ate the whole box. That night, the two of us almost came to blows over the bathrom. We had the worst case of the runs I had ever heard of.
I thought about my experience with the ex-lax and my experience with drinking and using. They were nearly the same. I was like pudding when it came to waiting for the restroom, just as I was like pudding against the drugs and alcohol.
With this explanation, I felt ready to begin the journey through the twelve steps.
The most wonderful thing about the journey through the twelve steps, that I have experienced so far, is that as you continue to grow and learn, you can see the changes in your life and in your feelings. When you write about it, you can read the difference in the way you think. I love the twelve steps, I love the shared experiences, I love the fellowship, but most of all I love the growth I have experienced.
I suppose you could say this is a commercial for the 12 step program I am working. However, I am not going to tell you which one I am working, nor am I going to mention the many that I know about. What am I advertising you ask? That is very simple. A program that works for you, because you can only get something different when you do something different.