It has nothing to do with the basketball. Nobody at ESPN (or America) cares whether the games are 72-65 or 118-117. It's the brand names and the personalities. ESPN knows how to market the Lakers and the Celtics because of their fan bases. Same with the Knicks.
The Bulls have Derrick Rose. The Heat are the villains with the electric fast break. OKC has two young high-flying talents. Even the Clips have Griffin and Paul.
With the Spurs there is nothing to market but great basketball. The brand is blah. The market is blah. There are no controversies. The players love each other. Everyone loves the coach. Nobody ever says anything stupid. There is no Manu vs. Tony, even though ST thinks there should be at times. Nobody ever commits a crime. There's no story line about so-and-so "finally getting his shot at a ring after 12 years" or whatever. Two of the players are foreign (news flash, the casual American sports fan is xenophobic as ) and Duncan hasn't said anything interesting in 15 years. There's simply nothing for the broadcasters and executives to sell.
Forget about the quality of basketball with the Spurs vs. other teams. Most people who watch aren't sophisticated enough to figure out the difference or to care. The Spurs won't ever be compelling unless they do something off the court that interests people. It's sad, but we're a tabloid culture. We're always going to have that "boring" reputation ON the court just because we're boring off the court. It doesn't matter who the Spurs play in the Finals, if the games are in the 80s or 90s, they'll be blamed for dragging the scores down even though it'll be the East team's style, not ours.