They are mainly produced by technically inclined people and hence their function takes priority over sex appeal - not good in today's fashion oriented marketplace. Also the public have no way to try out a recumbent and feel how incredible it is to sit fully supported on a bicycle. They do not know how and where to purchase a recumbent. Non-speciality bicycle stores will not carry them because in today's competitive marketplace it would be too expensive to carry an item that is not fashionable. It is very much a chicken and egg situation.
People have concerns also about the learning curve for riding a recumbent.
The high performance recumbents do take getting used to, though no different to getting used to a standard bicycle if starting from scratch, but transitioning does require a few weeks of concentration. It may surprise you to know that there are dozens of manufacturers of recumbent bicycles in the U.S., and many more in Europe. The huge popularity of the recumbent exercise bicycle in fitness clubs is a good indicator for the future. In conclusion, recumbents are more comfortable for long distance riding and provide better visibility with the head in a more natural position for looking forward. The typical unfaired 'street' recumbent will usually be as fast, or faster, as a good road bike on the level, faster downhill, but slower climbing hills due to additional weight and more limited use of different muscles.