Why is cannabis illegal but alcohol and tobacco legal in the UK?

why have the uk government banned cannabis even though its not as dangerous as tobacco or alcohol.? there are no advantages of tobacco and alcohol but cannabis has alot of advantages e.g. pain killer, cures asthma, used in other herbal remedies in some countries, a person cannot die overdosing on cannabis. etc. if its for money then why dont the government illegalise tobacco and legalise cannabis and sell that instead , im sure they'de make alot more money.