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.