Having the spare on the driver's side is a horrible idea in terms of safety while addressing a flat tire.
The only explanation that I can think of is that it was done for aesthetic reasons. If you were in charge of marketing and wanted to create an image which showed the friendly looking driver (as all Chevrolet truck owners which I have ever met are), and you wanted to display the latest and greatest snazzy looking spare tire option, you would naturally put it on the driver's side.
