You can thank the mighty United States of the A for it. In India my dad used to tell he went to government rationed cannabis stores to buy it. Then some stupid drug treaty made by the Greatest nation in the history of mankind came into effect making weed illegal overnight.
India is a bit beyond ridiculous and just shows when Asshole with properly big A (Nixon) is given enough power, he can fuck up most of mankind for few generations easily. Just to have more oppressive tools available against people not aligned with state (hippies, Mexicans). Also shows that people will generally show a big fat middle finger to government and keep doing what they want to do, but its a very useful tool for oppression and creates a lot of corruption surface.
As to why its such a ridiculous situation in India - as per my limited backpacker's experience marihuana is a sacred plant, used for thousands of years to celebrate god Shiva, one of main deities in Hindu pantheon. Seeing a sadhu (holy man) to smoke chillum is as regular sight as birds flying in the sky, I was once even given such a chillum within bigger group of sadhus on Ganges bank (I think in Haridwar).
Its like India becoming world dominating power and starts forcing every single christian country on the world to ban alcohol or using wine in religious ceremonies. And they actually implemented it for solid 6 decades so far, via United nations being a slave to US (and when its not, US presidents are quoted about kicking them out or simply ignoring its existence).
