Answer:
Because they lost the Opium War
Explanation:
At first, China didn't know the negative impact of Opium consumption and let the British government entered the market and sold the product freely to the people.
But soon, they know how addictive it is and tried to prevent the British traders from selling it to their people.
Since Opium sales was a huge source of income for British government, This made Great Britain retaliated with armed forces. China lost the war and agree to let Britain increase the sales of Opium to china as requirement to stop further attacks. This is why opium imports into china was increased.
