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.
Overnight? Don’t think so.
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.