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E, DanielFirst, everything everyone has already said.
Second, that shift you're looking for explanation of, which seems to coincide with the timing of the baby boomers is....... TELEVISION!
Guys younger than 45 like myself grew up with the tv as a close and ready companion, and though I read considerably as a kid (unlike many of my mates) there is no doubt tv and the instant information fix it provides became a real friend. Reading an old dusty book just doesn't entice in the same way that holding a card that reminds me of parts of my youth, no matter how rare and wonderful the tome.
Third, and feeding directly from second, is the fact that human beings take in information in ways they show preference to......Some are very visual, some auditory, others like to 'do' so as to relate and understand concepts. While we all do a little of each, the visual is most easy to stimulate and connect a majority in our world to, so quick ever moving stimulus like tv/billboards/packaging and tiny soundbites of content are what much of society has moved to.
A baseball card, with image on front and tiny bit of text on back, fits perfectly with our mildly a.d.d culture. Thus the current and likely future status of memorabilia versus cards.
Daniel