I assume you mean "partisan"? It's because the people who want it will still vote for you if you keep it criminalized. But a significant number of the ones who

I assume you mean "partisan"? It's because the people who want it will still vote for you if you keep it criminalized. But a significant number of the ones who want it to stay criminalized will totally write you off if you decriminalize it. In the US, how motivated people are is usually even more important than how many of them there are. Combine that fact with the electoral college and gerrymandering, and there is a constant threat of minority rule. EDIT: Private prisons. Can't believe I forgot about that. Everyone else who replied has got the main reason nailed.