I have noticed that many hybrid vehicles have little or no trailer towing capacity. For example, the Toyota Auris Touring Sports hybrid model has 375 kg trailer towing capacity (unbraked + braked) whereas the non-hybrid model has 450 kg unbraked and much higher braked towing capacity. The Auris non-Touring-Sports model in the hybrid variant cannot tow a trailer, and Gen1-Gen3 Priuses also cannot tow a trailer. The Yaris hybrid model cannot tow a trailer, but the Yaris non-hybrid can tow 550 kg unbraked and much higher braked trailer.
Why is this the case? Considering that in Finland, the maximum speed allowed for trailer towing is 80 km/h, and that you can drive continuously for 120 km/h with no trailer, surely the load trailer towing puts on the engine, inverters and motor-generators is lower than the load when driving faster than permitted with a trailer. Similarly, hybrid vehicles can be used in a hilly terrain. Driving up a hill is heavier on the system than towing a trailer on a flat land.
Fortunately, this seems to be changing. The Gen4 Prius has 725 kg trailer capacity (braked + unbraked).
Why is the trailer towing capacity on many old hybrids low or even zero?
Why is this situation changing now?