Chevy makes cars...like the Corvette, and light duty trucks
GMC makes trucks (light and heavy duty)
GMC has millions of dollars in government contracts for industrial trucks, chevy does not, and GMC has over the road truck hauling division as well.
Also, GMC and Chevy have a full infrastructure of parts suppliers that Saturn does not have. Close either one and hundreds of thousands of jobs are lost, part of the union deal was to keep those jobs. Saturn is a self contained apparatus with fewer jobs lost.
Sort of makes sense when you use all the facts...
PS...Saturn was not making enough money to justify staying open, in fact the other divisions were subsidizing Saturn