Because it is easy to decriminalize a product that is in direct market competition with tobacco when you have tobacco lobyists, isn't it.
Pot was made illegal here in the UK in 1974 thanks to a lot of pressure from the US Health department. Said health department basically made pot illegal thanks to pressure from the tobacco and oil industry (you can make amazing biofuel from hemp). The name 'marijuana' was given to pot to give it a dangerous edgy mexican vibe, so that straightlaced white folks would oppose it. They said pot made you listen to jazz music for christ's sake!
There is scant evidence that pot does any harm, and it certainly does not ruin lives the way alcohol does (you never hear "my Dad was a stoner, he used to beat us every Friday").
The problem is too many generations are used to pot being the dirty thing, which proves that people are gullible when faced with "evidence" from the government regarding the impact of pot.