Because in a locomotive, there are times when you need huge amounts of torque -- pulling a mile long train up a hill for instance. A combustion engine has to be sized to a torque load, and a combustion engine sized to maximum torque is just plain wasted mass, space, and cost most of the time. Fuel economy scales directly with displacement.
With hybrids, you can size the combustion engine to the average, and let batteries plus electric motors handle the peaks. This works splendidly in locomotives because there is a huge difference in the torque needed to pull up a hill and maintain speed on level ground.
Hybrids, aren't needed on the roads because there isn't that much difference between the peak torque and average needed. Small cars have already been sizing engines' peak torque close to the average. The mass of the batteries and motors completely eclipse any combustion engine mass/volume savings. And automobile engines are already very small and relatively efficient.