There are many things that will likely keep cards more popular (and generally therefore more valuable) than memorabilia:
- Nostalgia from childhood (the entry price for memorabilia is usually prohibitive for children, and you don't have nearly as much trading etc)
- Checklists give people obvious clear goals, knowledge of what to focus on etc
- Print runs and scarcity are often much more easily defined
- They store well and don't take up much space
All that said, there is so much more history and variety in collecting memorabilia. The range of options is nearly endless, and there is so much to learn in any area. For me, the areas are Game Used Lineup cards, Integration-related items, WWII baseball and Negro League items.
It was shocking and wonderful to discover that when I was able to chase down the 1st contract Roy Campanella ever signed with the Dodgers minor league team, I could purchase it for far less than a high-grade rookie card of Campanella. And there are few things cooler than a Program from the 'World Series' tournament played by soldiers (including HOFers Leon Day and Willard Brown) in Nurmberg Germany in 1945 with Hitler's complex in the backdrop after the defeat of the Nazis.
I've definitely found my focus has shifted away from cards. The exploration and learning is endlessly richer. The cost for pretty significant historical pieces is often far less than you'd imagine once you get past the more standard memorabilia (like a game-used bat or jersey from an iconic vintage player).
Of course, one of the casualties is that as you continue to learn more and go deeper, you find endlessly fascinating corners of the hobby whivh others have no concept even exists.