Reasonable people (like conservatives) realize that cigarette smoking is unhealthy. It makes you wonder why an ignorant selfish hypocritical faction of society would want to legalize a more toxic substance.
Reasonable people (like actual conservatives) would realize that the natural state of all substances is legal and unregulated. Marijuana was unregulated for all of human history before the United States began passing laws to make it illegal.
The government was enjoying the police power gained in prohibition and didn't want to give it up when prohibition was ended so they hung on to the marijuana laws they had and enacted even more.
Government legalizing marijuana is like government legalizing guns, or sex, or breathing. These are things to which all humans have natural rights and the government has no right, and in the case of the United States government no authority, to regulate or restrict at all. It's personal choice. The government can make certain behaviors criminal, such as murder, or robbery, or operating a vehicle under the influence of drugs, but it's the action that has a victim that is the crime.
What is not conservative, is to be an authoritarian, believing that government has the authority to force others to behave how you think they should behave; that's authoritarianism and has far more in common with socialism than with the libertarian attitudes of our Founders and the libertarian intentions of the Constitution of the United States.
On the first day of 2014, Colorado became the first state to permit marijuana dispensaries to sell pot for recreational use. Across the state, celebratory stoners welcomed the New Year by lining up at licensed retailers to buy bags of (heavily taxed) artisanal marijuana, with varietal names like...
origins.osu.edu
Millions have been killed in the war on drugs - not from the drugs but in the war on drugs, globally. Millions more are in prisons or have been in prison in the US alone, probably hundreds of millions globally.
Then consider the crime caused by the war on drugs but unrelated to drugs directly. When people learn to ignore stupid laws, unenforceable laws, they learn to ignore the law in general. Respect for the law erodes and people simply live their lives outside the law.
The war on drugs has done more damage to every country in the world than any other event other than communism/socialism. If there's one thing that the USSR, Russa, China, Cuba, Venezuela, Columbia, can all agree on, it's making drugs illegal solely for the power it gives to government.