I really don't think its arguable that baseball has fallen nationally. It's probably the 4th or 5th most popular sport option depending on the market. The NFL, college football, college basketball, and sometimes the NBA rank higher. That doesn't mean baseball is in trouble, it just means it's not the top dog it was 10-20 years ago.
Also, anytime it's a series for the championship it's not going to be great numbers anymore. Games 1-3 of a seven game series have little intrigue to the casual fan because no matter what there's still another game. Part of the reason the Super Bowl is such a big event is that there's no second chance, no reset button basically.
The fact this series is between the Royals and Giants also hurts the numbers. The Royals are a great story and much of the country is cheering for them, but that's not enough to get casual viewers, especially East Coasters, to tune in. I'd be willing to bet that before the playoffs, your average fan could only name a handful of players from both teams combined.