because it's a HUGE piece of revenue. Every single state could use more money for schools, roads, disaster relief funds, etc... The National debt is forever growing. You name it. Washington and Colorade have already figured out what's up.
Ok, the national debt won't even be dented by tax revenues from marijuana, but when congress comes to grips with the fact that it will at least help *some*, it will have to be legalized at the federal level for them to slap more taxes on it and take advantage. Yes, it'll be a hit to the wallet of the user, but federal legalization will make things better/easier all the way around.