Posted By:
MarkI could be mistaken, but I think:
i) baseball was the most popular sport in the early 50s (mickey, duke, mays...);
ii) due to baseball's popularity baseball cards caught on with baby boomers;
iii) baseball cards started becoming valuable in the 70s and 80s (and became synonymous with card collecting) because the baby boomers wanted to remember their youth; and
iv) people are passionate about baseball cards today because they are particularly valuable (It's easier to be passionate about a $5k invetment than a $200 investment - this might also explain the dearth of CFL or soccer card collectors).
I expect the relative popularity of sports will change over time - for example, Tiger Woods has made golf cards popular which I think makes some want to collect vintage golf cards. So perhaps football's popularity will eventually drive vintage football up.