Well, I'd wager that the majority of Americans don't even have MLB Network. I had to upgrade to another package that was $20 more per month to get it.
Also, hard to get really excited for a tournament when you're best players decline to play - what does that say about how important it is? Kind of rubs off on the fans a little bit. I think more people would be interested if you had guys like Trout, Fielder, Posey,Verlander, Kershaw, etc playing. They had to get Derek Holland to play for them, there's probably close to 50 US pitchers that are better than he is.
And most importantly, baseball means a lot more to these other countries than it does to the US, we have so much other shit going on at all times that it's hard to get a big entertainment giant like the US to pay attention to an international tournament going on. Typically, smaller or less developed countries are always going to get more excited for international tournaments like this, it's just simply a much bigger deal for them than it is for us.
I think the players from a lot of these other countries are a lot more passionate about the tournament and realize how important baseball is to their life as well. You've got a lot of players that grew up in poor countries and baseball was a way to make their life better for the future, IMO US players are more likely to take everything they have for granted.
Even team US doesn't seem to be taking this tournament all that seriously, Torre put Victorino in over Stanton in a must-win game against Canada so Victorino could get some at-bats to be ready for opening day.