I can think of two main reasons why it's not:
- Historical - both alcohol and tobacco were drugs introduced long long before the harmful effects of them were ever noted, therefore there was no need to make them illegal. To make them illegal now would cause absolute uproar, and uproot a huge amount of tax coming in (let's face it, now is really not the time to be thinking of sources of cutting income!)
- Monetary - As above really, the taxes would be minimal because people can grow the stuff at home.
- Message - Like it or not, legalising something sends out the message to all that this is a good thing to do with no ill effects on health or anything else. Keeping it under the radar and just not bothering to arrest many people over it sends out the message that "we don't particularly care if you do, but we're not saying it's ok". That's probably the message they want to send out.
- Health - The research into the long term effect of cannabis on the human body doesn't come close to the extent of the research done into tobacco or alcohol. This makes sense really, since you can't officially publish in a journal that you've been conducting an unauthorised study with an illegal substance. The true answer at the moment is we just don't know if, long term, it's more harmful than cigarettes or not.
I don't know a huge amount on the topic, but I'd say it's far from a dead cert call that it should be legalised - there's a lot of complicated political, economic, physical and even subliminal aspects that would all take a lot of time and money to look at in detail first. I'm not sure in the current economic situation that's really the best use of resources...