That's mainly because of the QB position. No position in baseball comes close to a QB as a far as impact on a game or season. I think the difference is any market can draft, develop and sign long term a top player in the NFL where in baseball that's less likely for many teams. The top talent is much more tilted to the bigger markets in baseball where it is more spread out in football.
The top 10 players of the NFL at the beginning of this season as voted on by players plays in KC, Cleveland, SF (2), Minnesota, Baltimore, Miami, Buffalo, Dallas and Pittsburgh.
MLB Network's top 10 before last season in baseball was Atlanta, Los Angeles (3), New York (3), Texas, Houston, and Seattle.
A team in baseball can catch lightning in a bottle like KC did in 2015. But the winners have generally been mostly from the larger markets and I don't see that changing. Especially now that big time talent is coming from Japan where they will almost never even consider anything but the biggest markets.