I think that as upright mammals, we are accustomed to seeing the world from the vantage point of bipedalism and we feel the most relaxed when adopting that posture. It is only natural that people prefer to remain upright when moving through their environment - whether on foot, or on bike, or on public transport. While upright bicycles may not be appropriate for sport or long distance travel, their importance in the realm of "normal cycling" cannot be overstated. Most people considering a bicycle for transportation do not see themselves as "cyclists". They want to be their usual selves, except on a bike. I think the ability to remain an upright mammal is crucial to retaining one's inherent sense of self while cycling for transportation.