People have been smoking marijuana since before recorded history. Yet it is here and now that all of a sudden the potency has risen dramatically.Name one horticultural product whose productivity hasn't increased dramatically in the past few decades. Horticultural science, whether applied to legal or illegal products, has come a long way. To me that seems a more plausible reason for the increase in THC levels in pot than whether marijuana is a legal or illegal product.
You don't get more nutrition out of a modern corn plant. You just get more resistance to bugs and weed spray. Marijuana, opiods, cocaine, and alcohol are different. These products have become more economically dense. You make more money from given units of weight and volume. This doesn't serve the consumer. For the consumer more potent drugs or alcohol simply means taking less to achieve a given level of effect. Increasing potency serves the smuggler only. It makes it easier to make more money for a given level of risk.
BTW, the higher levels of THC in modern pot comes mainly from plant breeding, not high tech. It could have been achieved at any time in history if there had been a market demand for it. But when weed was unregulated it was cheap and easily obtainable so there was no need.