The original parts packs failed becuase kits were two dollars. The Revell parts packs were 69 cents and required purchasing a body, frame, engine, pack with axles and suspension pieces, and tire pack. Almost twice what a full kit ran for. Only extra pieces were suspension bits. The double kits were an attempt to bring the overall price down. I don't think they sold all that well maybe Tim Boyd might have more info.
Yes the suspension pieces would be nice to have but I really doubt the sales numbers can justify the rest.
A good source of parts are the Roth outlaw, beatnik bandit, kits. The 29 ford pickup also has some great parts. Also the orange crate. Lots of kit bashing material.