Well, britbox, that may be part of it, but I think it goes deeper. Cruelly, it may be the spread of baseball into the more southern and western states that laid the groundwork for baseball to cease being the main sport everyone waited for during the cold winters of the midwest, mid-Atlantic and New Englander states. "The boys of summer" was how folks referred to baseball players, but now baseball can be played year round in warmer, sunnier climates, which just happen to have other things to distract the young folks--sailing, biking, basketball, football, boating and fishing. The baseball world used to WAIT for the coming of Spring training down in Florida, Texas, and whatnot and then teh game began in earnest with the coming of summer. None of that is felt much anymore and during the winter, the NBA has grown into an erstwhile alternative, and an exciting one at that. You got basketball now trumping hockey even--during winter!
On top of all that, baseball has lost what made it so special for those before my time, although it was largely the same way when I started playing ball in 1970--the equating of great baseball heroes with a particular team. Stan the Man Musiel is Saint Louis, just as Mickey Mantle was a Yankee and Ted Williams was a Boston man (even though that was love-hate oftentimes). Along came free agency and rarely do players stay with one team--through thick and thin--any more. That is why when guys like Ripken and Jeter step down, it stirs something in those of us who can remember the way it was. I am an unabashed sentimentalist and the nostalgia always gets me. I am so happy I have those sentiments, though, and feel a little bad many young folks may not ever feel that way about the National Pastime (for the USA).