It’s not because they aren’t being encouraged to get into pro baseball:
https://www.mlb.com/news/diversity-blossoming-within-baseball/c-224160762
...Over the past five years, the first round of the amateur Draft has featured 34 African-American players out of 168 total selections, or 20.2 percent.
In each of the past two Drafts, an alumnus of the RBI program and the Breakthrough Series (Dillon Tate in 2015 and Corey Ray in ‘16) was taken among the top five picks.
These prominent Draft selections have contributed toward African-Americans making up 14 percent of MLBPipeline.com’s 2017 Top 100 Prospects list, with Latino players accounting for 33 percent.
Black players account for 17 percent of the spots on the Top 100 posted by ESPN.com’s Keith Law.
Each of the top three and six of the top 25 prospects for the 2017 Draft, as determined by MLB.com, are African-Americans, including Hunter Greene, an alumnus of MLB’s Youth Academy in Compton.
And at the high school level, five of the first 12 players on Baseball America’s Top 100 High School Draft Prospects list are black...