The shit costs money. Most black folks don't have the monetary resources to afford a college education. Environments are a broad answer because no two colleges are the exact same. Usually, it's the influence of factors OUTSIDE the college environment, academically at least, that lead to dropping out.
Like them posters against alcoholism talking about how "it's ruined more aspirations that calculus, organic chem, and statistics combined".
"The reason most black folks don't have any culture is because it is out on loan to white people. With no interest." (c)Ralph Wiley