I presume that by "they" you mean the BBC.
Any retail release of a BBC production (or of any other company) in any format (including DVD) requires a distributor and it appears in the case of "Sleepers" that Acorn Media US, who have a history of providing quality British television to the US market, believe it financially sound to offer this title within the US whereas UK distributors (including Acorn Media UK) have not come to the same decision.
http://www.acornmedia.com/about/about.html