I think that this series simply lacks glamour. The Lakers are one of the league's glamour teams, as was Chicago during the 90s, and that brought a lot of luster and public interest to the finals. The Spurs, although they are a good team, lack any kind of glitz or glamour.
The Nets are virtually the same team now as last year, minus Van Horn, who isn't exactly a media darling. But viewership is far lower because people will tune in to watch the Lakers play pretty much anyone, but many people could care less about the Spurs.