When the industries were changing over from line shaft driven machines to individual motors, the change didn't happen overnight. A shop with an existing overhead shaft who wanted to add or replace a machine would be more likely to buy another shaft-driven machine than to buy a machine and an expensive motor. On the other hand, a new shop just starting up would be facing a huge cost to install a line shaft, so they would be more likely to buy a few machines with individual motors.
The point being, the manufacturer had to keep making both types for some time. It was easier and cheaper for them to just add a motor mount of some kind to an existing machine than it was to design a whole new machine. It was also cheaper to stock one machine and a motor mount than having to stock two versions of the same machine. With a bit of creative design, you could probably make the motor mount work for several different machines and lower your costs still further.