I think there's two parts:

1) A lot of car buyers worry about resale value. For the same reason a house with a purple and pink paint scheme won't sell at top price, a car with strong colors won't either. You might like seeing cars in a variety of colors like bright orange, bright green, etc., but those colors will absolutely turn away a significant fraction of the potential buyers, thus lowering resale value. If you could just press a button inside the car to change its color, it wouldn't be this way, but as it is, repainting a car is prohibitively expensive.

2) Just look at the way people dress these days. Boring colors are in, bright and bold colors are out and generally associated with the 1960s. We're in a very bad time now as far as color palettes and styles go.