GM is just too big for it's own good.

look at car companies like circles, imagine the area of the circle being the driving force of production, the internals of the company as you will. and now think of the outside circumference as the face of the company, the cars the dealerships.

as the companies get bigger they start to have more exposure, but that is overshadowed by the fact that their products start to overlap and compete against each other. as the circles grow the area inside the circle grow at a much faster rate than the circumferences can keep up with (such as a circle with a circumference of 2 will have an area of .3, where a smaller circle with a circumference of 1 will have an area of.07)

honda and toyota have successfully limited their growth, offering a wide range of products that generally don't overlap. (for the exception of the element which overlaps slightly with a CR-V, and now the Civic SI with the RSX type-s for honda)

for this same reason DCX is doing ok right now, becuase they have dodge as the brute, mercedes as the luxury, crystler is the odd one out, jeep is just jeep, and then mitsubishi is the economical fun company.