Its not just one reason. First off for the last 15-20 years, more than a third of the NBA coaches were well established with great to good coaches, who deservedly so have earned the right to continue to coach in the NBA. That list inlcudes or included, Jackson, Riley, Rudy T., Sloan, Popovich, Karl, Jeff Van Gundy, Larry Brown, Adelman, Nelson, to name a handful off the top of my head. Interesting enough Lenny Wilkens was in that group as well. So when a third of the jobs are already held by successful coaches it makes the hiring process a little tougher. Now throw into the mix guys like Carlise, Bird for a stint, Scot Skiles is back doing a nice a job who also has had previous success and then the jobs become slim pickings. Many of those black coaches really sucked though honestly. Darrell Walker, Randy Aires, John Lucas, Fred Carter, Magic Johnson, Isiah as well, just aren't that good. Even Paul Silas like some of the white guys who aren't that good have been around the block as well. But Look at Doc Rivers, Byron Scott, and Maurice Cheeks, all 3 of them will take up 3 coaching spots the next 15-20 years if they want. Nate McMillan is doing ok, not a big fan of his but he's winning. But if you don't win regardless of your color you will be fired, its just that simple in the NBA. Look what Nelson just did. He handpicked his successor who is black, a former player just two years ago, who to the best of my knowledge has never coached basketball in his life. So the oppurtunities are there just aren't a lot of available places. And many high profile black college coaches have been courted by NBA teams, they just didn't want to leave the college ranks. Guys like John Thompson, John Chaney, and Tubby Smith has been rumored a few times as well. But the NBA weeds out bad coaches regardless. Just ask Rick Pitino and Chris Ford as well dozens more white coaches. I think the NBA is doing a good job of diversity. Now the NFL is totally different story, because the same bad white coaches keep getting hired over more than capable black assisstants who deserve the chance. I think the NBA is alright and getting better.