Location, location, location
A declining market share means that GM doesn't need nor want as many dealerships in close proximity to each other. Imagine if people starting eating half as many hamburgers - McDonalds wouldn't want its golden arches on every street corner. That would be saturating the market, and they'd actually be competing against each other, not the competition, and driving down prices, and reducing profits.
As for the wisdom or stupidity of GM's business decisions, this one actually is pretty sound. For a change.