I'm not sure if it's actually STRONGER, but people have spent decades trying to make a balanced strain. Like breeding the plants so they are potent but still have a lot of yield. Also, I wonder what kind of conditions people grew Cannabis in back in the 1970s? Perhaps over the years people have also perfected ways of growing it, too. So now that I think about it, the potency hasn't increased technically... people have just found better ways of growing it, so the plants are healthier and produce more THC. So it's not that Cannabis is actually stronger, it's just that people have found better ways to grow it, thus improving how well the plant has grown. I hope that made sense.

Edit: Yes, I know the last few sentences are redundant, but oh well.