It's not just hometown bias. San Antonio and Portland are the top two teams in the West. The Clippers are good, but the Knicks, despite some recent wins, are a team with a losing record.
Of course, anything can happen in real life, but theoretically at least a game between top two teams would likely be more riveting than a game between one good and one crappy team. IMHO, the only compelling reason to show Clippers/Knicks on national TV is their home cities. If teams with equal records from Sacramento and Cleveland, say, were playing, the game would not be nationally televised.