Unlike the NFL, post-season ratings can make up a considerable portion of the ratings for MLB, and therefore ad revenues, for networks that have broadcast contracts, as is currently the case with News Corp's FOX network. FOX recently inked a new deal that starts in 2007, extending their relationship with MLB.
TV sports executives start to look for what match-ups will come out of the regular season into the playoffs, and with that try to determine if ratings will be high or low. As the Wall Street Journal reports (free for one week from publication date, then subscribers only):
More so than with other major sports, Fox bet that money on what happens when the regular season is over, especially on the World Series. Not only are some matchups more attractive than others, but a series can flame out in four blowout games, or stretch for a sublime seven. A bland, short series means fewer ad sales and a poorer platform with which to promote the network's new prime-time entertainment lineup, which will begin airing after the monthlong orgy of playoff baseball.
"Probably 75% of the economics are wrapped up in the postseason," said former CBS Sports president Neal Pilson, now a sports television consultant.
Football, by contrast, gets big ratings all season long, and in the postseason, interest is always high, no matter who is playing. Some Super Bowls get bigger ratings than others, but the sport is truly national and the range is narrow. Baseball, more regional, more tribal, is prone to much fluctuation. World Series ratings sometimes swing by nearly 30% from year to year. In 2004, the Boston Red Sox and the St. Louis Cardinals were a hit; last year, Chicago and Houston produced an all-time ratings low. While those cities are large television markets, the Astros and the White Sox didn't have a national following and Chicago's four-game sweep didn't help, either.
As noted in the article, the percentage of households that tune into the World Series has dropped since 1981. Recall that having a Yankees v. Mets World Series isn't a recipe for successful ratings. The Subway Series in 2000 garnered the lowest ratings ever up to that point.