My opinion on one of the reasons for that is the lack of consensus for some of the prewar rookie cards. For example, Ty Cobb has something like 5+ rookie cards. If there is consensus and some set rules that collectors can agree with, then I think they would be more popular for prewar. For example, modern cards often have multiple rookie cards for a player also. However, I think the rule is something like the most valuable card for that player with a print run greater than XYZ. (That way certain short prints won't be included.) For example, I think right now, there is general agreement among that M101-5/4 is Ruth's rookie card, so auction houses have commonly advertised this when selling these cards. Same thing with the 1925 Exhibits Gehrig (sorry, Adam).
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