As I grow older I see more of my friends trade in their two wheels for 3 due to comfort, reduced physical abilities, technology improvements and increased caution that comes with age. There are some beautiful trikes and 3 wheeler’s being built today, but they don’t have the same purpose as other designs earlier in motorcycle history.
At the turn of the century it was a different set of circumstances that caused the creation of 3 wheel machines. Remember that as their name implies “motor-bicycles” evolved from bicycles. When an engine was added, these “motor-bicycles” became heavier and more difficult to ride. The 3 wheelers were a solution to the problem. Initially two wheels were placed on the back, however a better solution (at least at the time) placed the 2 wheels in the front. This made it easier for manufacturers to mount passenger seats and cargo boxes on the front. Additional configurations placed the engines and drive trains everywhere on the frame and connected to both front and rear wheels.
Also, around this time the automobile was starting to evolve into a product for the masses, however it appeared to be much less expensive to own a motorcycle or 3 wheeler as a family vehicle.