Granted, recumbent trikes & bikes look a bit strange to anyone that are more familiar with the DF upright bicycles. They are not new and have been around for many years. What very likely happened to slow their popularity was when in 1934, they were prohibited from racing with upright DF bikes because they were so much faster. Much less wind resistance and the power of the legs is used with greater efficiency.
Trikes are not great on hills but there are ways around that. Easiest thing to do is simply stop in place and there is no fear of falling over, even clipped into the pedals. Can't do that on any upright bike. My HP Gekko FX has an additional 3 speed rear hub that gives me 81 total gears. There are other options with that such as a Rohloff hub. Battery power is also easier to convert to since there are many places to mount the battery.
As many of you may have noticed, a trike is a bit larger than an upright DF bike. But many trikes fold in half and can be placed in the boot or trunk of even the smallest cars.
Many have found out to their great dismay that riding an upright DF bike for any period of time simply hurts. The saddles are narrow and HARD. Your wrists, elbows and shoulders take the brunt of the forces from any road imperfections and the neck begins to hurt because you have to bend it back and up to see anything. Those are the main reasons that many give up on riding a DF bicycle.
Recumbent trikes, on the other hand, take all that away. Many refer to them as mobile recliners as they are very comfortable even for hours upon hours. It really is like sitting in a chair as you are much more upright. Your hands simply rest on the handlebars and guide easily. Road vibrations do not travel up your arms. On my 2,600 mile tour there were many days that I did ride 8 or more hours. At the end of those days, nothing hurt, didn't have to stretch to get the kinks out. Even after my campsite was set up, I usually used the trike to sit on as it was much more comfortable than the picnic table.