1. Pot is a major economic competitor to beer and liquor, even as it remains illegal. Legalizing would only make it more so. Not only are the government and beer corporations buddy-buddy, but some state governments operate liquor stores which abide by certain federal rules.
2. There is a way to make it taxable, and that is to sell it out to big tobacco. Pot's also somewhat of a minor competitor to cigs.
3. There's really no reason not to legalize it based on the tax argument.
4. Another argument is that the USA has a drunk-driving pandemic (which is not true, but sensationalized by pain in the ass groups such as MADD) and that legalizing pot would only contribute to DUI and DWI crime statistics. This is true, sure; stupid people would get stoned and drive. But we don't have a drunk driving "problem" in this country really, drunk drivers are few and far between in most parts of the nation, and cops are dilligent about nailing them, and the laws are harsh. People are going to be stupid and drive, regardless of the substance.
5. Terrorists sell pot, and your $20 goes back to the terrorists so they can buy bombs and fly planes into things and annoy the Israelis. The government puts out this kind of propaganda, which is amusing. I think this is the only reason they're holding out on legalization - if drug money funds terrorists, then the government would be funding terrorists by proxy by allowing people to buy drugs (from the terrorists.) But the government directly funds terrorists. So this doesn't hold any water, either.
6. Pot was originally banned in this country (and this is true) because government members at the time argued that white women would get stoned and have sex with blacks. Again, true story. It was this arguement that got it banned.
7. Pot was banned for a "second" time because it was argued that it has no medicinal value. This is now proven, for the most part, to be false. However, it does not have enough medical value to a majority or even a relatively large minority of the population to be used as a valid argument for broad legalization. Pot has medical value. The drug Schedule is meant to ban drugs which have none. It's circular logic to say that since pot has some medical value that it should be removed from the schedule all together, though, for the above reason.
8. Pot dealers don't ID people that buy dope, therefore it winds up in the hands of our children. Yeah, but so does anything. Anybody ever pay a hobo to buy you a box of beer when you were underage? People are naive.
9. Can't promote "degeneracy" because this is a moral, upright blah blah society. More crapola put out by the government.
10. Alcohol is protected by the constitution. It was banned, but repealed. The 21st amendment (I think it is) is sort of like the second, but with alcohol instead of guns. Pot isn't. Bringing back prohibition would be unconstitutional - but drugs don't have that kind of protection.
On and on.