There's a reason. Cyclists began to see the merits of seat & back comfort, fun of steering with sticks and being close to the ground. There's more. A triker can climb with zero thought of balance. He can stop on an incline and simply sit comfortably to rest. A trike loaded with camp gear can manage steep ascents at low speeds --I'm talking 1 mph--and not be concerned with tipping over.
It's true, most trikers can't keep up with DF bicycles on climbs but get outta the way on descents. On flat terrain, trikers do keep up with DFs. I do, anyway.
The sticking point with buying a trike is cost. They're up there. Of course, one can find used trikes for sale which in itself can be exciting.
The next time you encounter a triker, see if you can have a chat. What he/she says can raise your eyebrows.