As a American and somewhat Cricket fan (my other half loves Cricket) I can see why American just are not in to cricket for the most part.
First you have the weird format of Test Cricket and ODI. If you think about the popular sports in America in terms of watching most dont last over 90 mins and baseball, which people here think is too long, is trying to be 165 min in length. So the idea of watching a game that last the whole day or about so, is just crazy to most.
20/20 seems to solve that problem, and I think with the faster pace of scoring also helps. As American love the long ball =P
And 2nd and most important thing is History. Yes History in our very young nation, but baseball likes to stick to its history and its place in American history. Its not called the "National Pastime" for no reason. Baseball was the first sport I learned as a kid and I think i am not the only one. So it would be hard to break the mold you learn as a kid about the sport, and since cricket is close in many ways to baseball it would be hard to unseat it.