Im pritty sure I was reading an article or someones post on a form although I dont remember which one, but he made a good point:

-The government wants pot to be illegal because it's hard for them to make money off of it. Some people think that marijuana is illegal because it is 'bad' for you, if that was the case then ciggarettes and alcohol who be banned to, but they aren't because the government can make money from them.

- Its relitivly easy to grow and distribute marijuana on your own, tax free. The government would have no way to know your information or keep track of distributers and manufactors to force taxation. Thus.. they made it illegal

I don't know about you, but it sounds like a pritty good reason to me. There's probibly many more 'reasons' but this one sure made a lot of sence to me.