To use an illustration that I've heard about a thousand times in KHs on the topic of raising a child:
Think of it like holding a spring between your fingers, if you hold it tightly then release it all at once, it's going to fly off and you'll never find it. If you release it slowly, you'll be able to hang on to it.
The application is to slowly give your kids more freedom so that they don't fly off the handle when they turn 18 and leave the house. It applies just as well (if not better) to the WT's system of control - keep people so completely repressed and make sure to fill their lives with lies so that if/when they can't stand the control or become aware of the lies they're going to throw away all the good things they've learned and fly completely off the handle.
Add in the constant indoctrination that you can't trust yourself, and that you're not good enough no matter how much you're doing and it's a recipe for someone to wind up completely lost, depressed, and without direction - which can often lead to substance abuse if someone doesn't have proper coping mechanisms. Of course JWs are also prevented from developing coping mechanisms because the answer to every problem is "the new system" or "wait on Jehovah" or similar.
The result is someone with a deep longing to control their life, but doesn't trust themselves to do it. They feel like everything in the cult was a lie, so they throw the good out with the bad. This leaves them with no direction, and no trust in themselves to provide their own direction, and no coping mechanisms for the stress this causes (not to mention the stress of losing their family and support system). If you put someone in a place like that, it's no surprise if they end up turning to drugs to cope.