Why do the majority of people on an ebike ride at a fairly low cadence?
There are (at least) two reasons. First, most (but not all) E-bikes use a control system that multiplies the amount of force or torque that you put into the pedals or cranks. Since power is the product of pedal speed and pedal force (or torque), increasing the torque allows the rider to reduce the pedal speed -- that is, the rider's cadence. Electric motors (can) produce full torque at any rpm, so these controllers typically modulate the torque output rather than the rpm.  

Second, if a rider on an E-bike were riding at "normal" cadence for his or her speed you would be less likely to notice the rider, so your observational sample is likely to be more heavily weighted to those with lower than normal cadence.