I'm sure someone will correct me if I'm wrong, but I've heard it has to do with the mass of the moving parts. Since each increase in unit velocity increases the kinetic energy exponentially (squared), the connecting rods, etc would have to be so huge to withstand the force that it's easier to optimize for low RPMs.

It's amazing to think how slow the pistons move on this engine. At 87 RPM, it would take a little less than a second for the piston to complete a cycle. Compare that to your average automobile where the piston completes 50 cycles every second (at 3000 RPM).