The only reason I can think of is size. There were dimension changes in '88 and '94. The '94 models are taller and wider. We love our '94, and at 6'4", I appreciate the extra headroom. The years you mention used more plastic in the interiors too. We've got oak cabinetry, and it's very nice. Plus, you can get a very nice '94 or later for much less than new, although they are harder to find.
'94 Excella 1000; '03 Silverado 2500HD