It's not as cut and dry as you make it sound. Yes, MLB can do better in promoting star players like other leagues do. But there are a lot of factors at play when you consider who you are marketing to.
It should be marketed to those who will never be 6'7" with a 40 inch vertical, or can run a 40 in 4.25 seconds. It should be marketed as a sport where it doesn't matter how big or fast you are, but by how you play.
Honestly, Trout and Betts are the perfect players to market that, seeing as how they are so diametrically different to one another in stature. If you are referring to marketing to the black community, two of the single most important black athletes in the history of sports happen to be baseball players. Jackie Robinson and Curt Flood.
One paved the way for black athletes to play major sports, and the other opened the door to make sure said athletes get the money they deserve.
Again, I really think that MLB has dropped the ball on this.
2 of the single greatest athletes that I have ever seen in my life, Bo Jackson and Deion Sanders, absolutely loved the game. Can you get more iconic than that ?