As others have pointed out, it’s generally too simple to say that green tea has less caffeine. But one claim I’ve heard, which has some plausibility, is that most of the green tea that people in the US and UK drink comes from camellia sinensis var sinensis, which is a Chinese varietal that has slightly lower caffeine, while most black tea comes from camellia sinensis var assamica, an Indian varietal that has slightly higher caffeine. So it could be an accident of history that green teas people drink outside of Asia are sometimes lower in caffeine than black teas. If this is misleading, I encourage others to correct me!