I think you guys are a little bit confused. There has ALWAYS been high potency cannabis, and low potency cannabis. 20-30 years ago there was LESS high potency cannabis "available" than low potency cannabis, so the AVERAGE was relatively low. Nowadays, the AVERAGE potency is significantly higher, because there is MORE high potency pot "available" than there used to be.

It is hypothesized that this is because of improved growing techniques, which may or may not be the case, but as far as the AVERAGE THC% increasing over the past 20 years, this is indisputable.

Marijuana more potent than ever | Booster Shots | Los Angeles Times