Market share?
In a tough harsh world, Christianity provides comfort. The poor tend to be treated the worst.
Christianity keeps you facing reality and addressing problems. Misuse of drugs can be like putting your head in the sand and not facing reality and not learning problem solving skills.
Christianity provides power, misuse of drugs weakens.
If you want to push dope and be the most successful, you need to wipe out the competition. Christianity is one of your competitors. So you try to knock it out of the ring.
Similarly if you have set up a false religion and want to be the most successful, dope (legal or illegal) is one of your competitiors, so you have to try to knock it out of the ring, along with Christianity.
If you need people to lay miles of cable, put up scaffolding or drive diggers, they've got to be able to calculate. And you don't want them falling asleep or how can you indoctrinate them? Let alone drive them to work endlessly for you.