It's the money.
The healthcare industry is worth Billions a year to many chemical and pharmaceutical companies.

In 2013 there were only 17 states in the usa allowing medicinal cannabis. In those states, only a small portion of the population used cannabis regularly, despite it still being federally illegal.

Just those few states, with those few people, saved the federal govt almost $165M in JUST prescription medication costs, this isn't counting private insurance and personal prescription purchases. When it's fully legal and the entire population is "again" using cannabis as a medicine, we're talking 10's of Billions of dollars a year in reduced spending in the healthcare industry. JUST for prescription medicines.

Home use of cannabis DRASTICALLY reduces medical costs.

In the USA, there's also the number of pension plans which are invested in private prison profits. Can't let all the cannabis users out, profits would tank. Do they have that issue in the UK as well?