Unless you are a real stud in baseball it takes much longer for players to fully develop. In basketball and football people can come in and make an immediate impact. That leads to a lot of money upfront, and if you notice, both basketball and football have very publicized drafts and systems in place where future stars get shoe contracts upon entering the league. Most players in baseball mature in their mid to late twenties and last about ten years. The draft doesn't make the big headlines nor do the players get $100 million dollar shoe contracts.
I think there's a number of reasons why baseball players take longer to develop. First of all, it's not exactly easy to hit a 97 mph fast ball over the fence, nor is it easy to learn to get a good hop on a deep ball, nor is it easy to learn to play the more specialized positions like catcher, SS, or pitcher. To be a successful baseball player, you also have to learn situational hitting. Baseball, especially in the NL, is very much a chess game the same way football is, but most of the script is written by the players on the field, not the coaches in the box and the QB calling the audibles. Good base stealers and hitters are given the green light and make good decisions accordingly. Good catchers and good pitchers call the game and the pitchouts. Also, I think its somewhat misperceived that baseball is an individual sport, and this is not true whatsoever. This is what makes baseball a sport in my opinion.
Like someone said, it takes a lot of instruction to learn the mechanics and situational hitting. It's an economic issue in that it costs good money to equip a team and bring in good coaching, its a brain-drain issue in that most good players raised in inner city neighborhoods don't return to teach skills to the next generation, most of all, a HUGE issue is that there aren't many baseball fields in the inner city. It costs more money to develop a baseball field, and a baseball field cannot be converted as easily as football/soccer/track fields.
I think it is also cultural. Not only to African Americans see a better cashout with basketball and football but they are more inclined to play these sports because they are more popular in the African American community.
Someone brought up European players in the NBA that come “a dime a dozen.” To be fair there are some good European players, but I think the consensus would say that American basketball players are still the best in the game. In baseball however, players from other countries play as good if not better than American players. Also if you read the news, you hear regularly of players from the Domincan Republic and other Latin American countries re-investing their time and efforts in their home cities and countries. Low Cost / Free training by ex-professional athletes will leave the next generation much better tooled.
Also like Jomodo said there is a correlation in the sport with parental involvement and success. I played on teams from a diverse middleclass neighborhood. I don't remember exactly but our racial makeup was probably 6 white, 4 latino, 1 asian, 1 african american. We beat up on teams from the ghetto but we also beat up on the rich kids as well because our parents kept us on the field hours at a time, got us together during the off season to throw batting practice, etc. etc. It also helped that we had 2 baseball fields.
Anyways that’s more my 2 cents than me claiming to know my facts. I’ve worked at inner city schools and inner city neighborhoods. You’d be surprised how hard it is to find a baseball field or a batting cage.