I agree with Tremont's post, but there are two additional factors which contributed to NHL level ratings for the NLCS.
-All of the games were 10pm first pitches. That's pretty late for us east coasters, who make up a large part of the viewing audience.
-NY bias. The Yankees first round loss and the Mets historic collapse, baseball left a bad taste in the mouth of New York baseball fans and thus a large number of baseball fans have tuned out of the playoffs. Still salted up from the Mets Game 7 loss to Cardinals, I did not watch an inning of last season's World Series. Similarly, following this year's Mets choke, I have not sought out postseason baseball. I've only watched a few innings here and there and apparently I'm not the only one.