Legalised gambling may keep some fandom national. Fantasy baseball has kept MLB from completely losing its nationwide following, but the numbers continue to trend down. Baseball does very well regionally, as large numbers of people follow the local team at least a bit, buying merchandise, attending an occasional game with friends. But no one is watching the game of the week anymore. People in St Louis don't care anything about a game between Houston and Baltimore. It's more like the old days before TV baseball really caught on, and the sport was played on the radio and in the newspapers, both of which were local. When there were 8 (or 10) teams in both the American League and the National, Boston didn't much care what happened to Milwaukee, even though the Braves had started in Boston. They were in the NL, and no one noticed. A few transcendent players were national, the Willie Mays, Hank Aarons, Bob Gibsons, and Sandy Koufax's know to the AL, the Mantles, Berras, and Williamses in the NL cities. Not much more.
As the amount of available baseball on TV exploded, so did a more national awareness of the sport. People started following possible record-breaking streaks elsewhere - Pete Rose, Cal Ripken. Home runs and steroids became a story everyone could understand, then faded. The stories of the Red Sox and Cubs never winning, or to a lesser extent, the Phillies' and Indians' woes were stories that people in other places could understand. Then Theo Epstein broke the evil spells over Boston and Chicago, and that story went away.
The statistics-lovers still keep track of what is happening around both leagues, but even among them, only the most intense. Even I look at the National League leaders for various statistics and have no idea who many of the players are. Nor did I even recognise all the names in the Yankees lineup last night.