American Jews just aren't religious. The majority of American Jews don't identify with their religion at all, being entirely secular. The result is that being a Jewish American is just like being an Italian American or an Irish American. If Jews don't identify with Judaism then they become just another minority that is going to assimilate into 'modern culture'. In fact some Orthodox Jews see this as the 'Silent Holocaust', that assimilation is drastically reducing their numbers.
This has been a steady process ever since European Jews decided to emigrate to America in the 18th and 19th centuries. Generally speaking, most decided it was wiser to adopt American customs and try to 'fit in' rather than stand out as a visibly distinct group. They felt safer that way, it was easier and more attractive to blend in and be protected the Seperation of Church and State.
Again, it was easier to cling to a secular American than to Judaism for many families. If an employer requested that a man work on the Sabbath, it was easier to comply than to worry about how he's going to feed his wife and 5 children. Nearly the entire continent was 'trefe land' or unkosher. So, it was easier to just eat what was abundantly available, even if it did not conform with Jewish Dietary Laws. So, one by one the Jewish customs slacked and then failed. Once you stopped keeping kosher, it was hard to justify following the rest of the customs. It was unthinkable, and even 'Un-American' for any group of immigrants to seal themselves off rather than take advantage of this new Land. This is how the faith of many Jews declined in America.
Some have even said that the concept of American itself is not conducive to Judaism, since the Constitution was based on the Enlightenment Ideals of individuality and freedom of choice. This is like poison to the communal spirit of Judaism. In America, supposedly everyone has the freedom to pick up or drop a religion as they see fit, individuality, right? But that undermines the concept of an eternal covenant to a nation, or a people.
Today it seems like most Jews have replaced the Torah with the U.S. Constitution, they replaced the Tanakh with the Bill of Rights.
Instead of relying on G*d for protection they rely on the First amendment, which says they must not be treated differently because they are Jewish.
I think this is a sad state of affairs. I know most americans would rather be inspired by the faith of the american jewish community than live in a totally bland, secular society. I wish history had turned out differently, that Jewish immigrants going into America had resisted American culture, like Belteshazzar resisted Babylonian culture, I know things would have worked out better.
There are many books on this issue, such as
The Vanishing American Jew
and Jews and Judaism in America