Motorcycles put tight limits on the lighting equipment they can carry. There's a lot less space for lights than on a car, and each gram of added weight negatively affects safety by raising the motorcycle's centre of gravity, destabilising the bike's handling and bringing a greater tendency to tip over. Motorcycle batteries and charging systems are likewise generally small and of low capacity. And can be impractically expensive to specify and make lights optimised for motorcycle service; motorcycle production volumes are much lower than car and truck volumes so each unit costs more, and expensive lighting can drive a motorbike's price out of the reach or will of a large proportion of buyers.
So for decades, motorcycle lights have tended to be adopted and adapted from the low end of mainstream car and truck practice. The old H4 bulb, which is at the lower limit of what's considered acceptably performant for car/truck headlamps, is several steps up from the bottom rung in the 2-wheeler world. The luminous flux from an H4 seems extravagantly high in comparison to that of an HS1, R2, or HS5—all bulbs widely used in motorcycles round the world.