I believe the NBA must take a lot of the responsibility. Ultimately it is driven entirely by money and the amounts they ask for broadcast rights means no free to air can afford the license (well one of the big ones could, but its not worth their while).
The same has happened to the EPL over the past few seasons. SBS hasn't been able to afford the rights, which really sucks for the non pay TV people
.
If they made their licenses cheaper, they would have far more exposure and popularity (although it seems the EPL doesnt really need it)