That’s not entirely true. There’s a long and enduring tradition of religious and recreational use of drugs in India (cannabis, charas, opium, datura). It was the British who applied their misguided Victorian morality on indigenous use.
It seems probable that being close to China in a variety of ways (geographically, socially, etc) may have allowed the Opium Wars to induce anti-drug sentiment in other Asian countries as well. There are probably also other reasons for it, but I'd expect that to be one of the reasons.