For the obvious reason why 3 wheel vehicles have never become mainstream.... If you work with computers you should know about Single Point of Failure. If you are cruising on the highway and get a flat with 1 of your 4 tires it's no Big Deal. How do you think that would figure with only 3.
Or another example. Why so many wheels on an 18-wheeler. Wouldn't it be awesome if a truck could do a u-turn in it's own length? Those things blow out tires all the time. But it doesn't matter because they have 18. SPOF. 3 wheel vehicles will never catch on in the mainstream. They'll only ever be the equivalent of glorified motorcycles - the ultralights on the road.
The tadpole configuration has stability issues though, rolling it over is fairly easy if the vehicle has a high center of gravity.
The Reliant Robin solved that for the most part by reversing the arrangement putting the single wheel in front, but they still tend to overturn quit easily.
The accident mentioned above during the demonstration probably really was caused by that other car, but the ease with which a three wheeled configuration turns over most likely contributed to the severity, and in the long run would have had to be solved using tricky engineering such as a single wheel for low speed maneuvering and two wheels for higher speeds.
Here is a Reliant Robin doing what it does best:
I think there is a topgear episode where they try to improve the Reliant Robin by putting training wheels on it.