The same reason GM had a chevy, pontiac, olds, buick, and cadillac. They covered the whole market that way. The cheapo (chevy) was basic while the snazzy (cad) had all the bells and whistles. Back then chevy and GMC were different trucks with the idea of capturing two segments of the market. It's called marketing. If the chevy had everything there would have been no market for the others.
