Recent recumbent converts often ask that since recumbent bikes are so obviously superior to upright bikes, why aren't they more popular these days? Here's one fairly plausible theory:

In 1933, a Frenchman named Marcel Berthet rode a recumbent bike to a new speed record of 50km/h. A furor arose in the UCI (Union Cyclist International), the general governing body for bicycle racing. After some debate, the UCI decided in 1934 to outlaw recumbent bicycles from competition, after which they fell out of favor with hardcore racers.

It should be noted here that the current human powered land speed record of 110.68km/h (68.73mph) was set in 1993 by a rider on a recumbent bike.