New strains develop with more of the substance that gets you high, and new strains develop that serve other purposes. Better technology means better, more healthy plants, and new strains that give you a better high. Plus, street marijuana can be very unsafe, and is fairly often cut with other drugs to make it addictive. Legalize it so we don't have this problem, and make a ton of cash like we did from alcohol
Modern plants have developed new techniques to make strains more potent than in the past. There are also newer delivery systems that make the intoxication of the same strains more potent. It is natural that technology has improved this product. More potent varieties of the drug make it more enjoyable.
The idea that marijuana is more potent today than before is a framing move by anti-marijuana advocates; by convincing older generations that drugs are more potent today, it allows them to tell their children confidently that they shouldn't do drugs while absolving themselves of any guilt of hypocrisy. The highest end marijuana is much more potent, but rarely consumed by casual users.