The people on this forum love bicycling and we ride for that love.
However, many people simply do not "LOVE" to bicycle.
For the majority of people, the motivation to bicycle commute would be for economics and for lack of a better alternative.
John E points out that bicycling in China (THE bicycle kingdom) is rapidly giving way to the automobile. JohnE is absolutely correct. I witnessed this sad evolution through 12 years of regular travel to China. Still, my Chinese friends tell me that they don't really miss the bicycle. They say "Mike, you bicycle because you like it and have convenitient alternatives. We bicycled all of our lives because there simply was not any alternative - rain, snow, or shine.
In the USA, the two biggest bicycle booms were in the 1890's and in the 1970's. In the 1890's, the bicycle was one of the best and most convenient forms of mobility to the urban dweller. He had access to streets/roads and horses were not available to many urban dwellers.
In the 1970's, gasoline prices doubled, inflation was rampant, and incomes plateaued. Many people took to bicycling for economic reasons. Of course, with the "back to nature" movement of the day, bicycling was quite fashionable and sexy, so others were motivated to bicycle just for the fashion of it.