Lighter motors of similar power operate at many thousands of RPMs, necessitating multi-stage reduction gearing to come down to useful speed/torque ratios. All these high-speed moving parts are noisy (often very), prone to wear, and can't be back-driven so require yet more wearing/moving parts (a clutch) to avoid being effectively one-way. Lighter motors require good airflow, cooling fins etc to avoid overheating when working hardest, which is when bicycles are moving slowest (loaded climbing), generating the least airflow.

But mainly it's noise. I can't stand noise on a bike. I designed the system to please myself first of all, to meet our family's car-free transportation needs, and quiet, reliable simplicity won out over the desire to have the lightest possible system.