Why not Impala?

[427TJ]

Another explanation might be that the full-size was considered the flagship car or the mainline car. All other models (Camaro, Chevy II, etc.) being sub-models. You could think of it as a holdover from the days when Chevrolet only offered one design (say, 1940) from which the various models (wagon, coupe, sedan) derived, all using the same forward body design.

Still clear as mud?

I guess the first break with the "mainline" model tradition would have been in 1953 with the Corvette and then again in 1960 with the Corvair.