The genetics of the plant is a factor. In the fine tradition of the American can-do spirit, even hippies were experimenting with selection for potency. Europe wasn’t as hampered by illegalites. Weed has been selectively bred for a lot of different characteristics...yield, variations in photoperiodicity to match plants to the latitude they’re grown at, appearance, characteristics that make them suitable tor indoor growing. Horticulturists love that stuff.
I've had good luck with common modern photoperiod strains, and good luck with autoflowers. Lately, electricity in my area is just too expensive to compete with the dispensaries.