This is true even if our definition of "religious" is minimal, i.e., observant of any specifically Jewish religious laws, attends synagogue once a month or even declares a belief in God.
According to a 2003 Harris Poll, "Only 16 percent of Jews go to synagogue once a month or more often"; and regarding belief in God: "Protestants (90 percent) are more likely than Roman Catholics (79 percent) and much more likely than Jews (48 percent) to believe in God. Religious affiliation here includes many people raised as members of a religion or religious group, regardless of what they practice or believe now."
Why most contemporary Jews are irreligious, given that the Jews gave the world the Bible and introduced humanity to the God of monotheism, is a fascinating subject. It is also a vital subject given the role that secular Jews -- such as Marx, Freud and Einstein -- have played in forming the modern world.
One reason was traditional (Orthodox) Judaism's inability to keep most Jews religious once Jews were free to leave the ghettos and shtetls (small Jewish towns or villages throughout Eastern Europe) in which most Jews lived.
The only Jewish religious alternative was a new Jewish movement called Reform Judaism, begun in Germany in the beginning of the 19th century. But with all the good intentions of Reform's founders to stem the departure of Jews from Judaism, Reform retained little that was distinctively Jewish. It dropped kashrut (the Jewish dietary laws), Hebrew as the language of worship, Jewish peoplehood, opposed the return of Jews to Israel (Zionism), and allowed moving the Sabbath from Saturday to Sunday.
By the mid-19th century, some Jews broke away from Reform and founded Conservative Judaism, in order to "conserve" Jewish religiosity without being Orthodox.
While Reform and Conservatism appealed to many Jews, a deeply religious, God-centered alternative to Orthodoxy that can keep Jews religious has not yet arisen.
And why did most Jews reject Orthodoxy? Over the course of thousands of years, a combination of anti-Semitism and Orthodox Jewish law -- one of whose primary purposes was to keep Jews separated from the non-Jewish world -- kept Jews in isolation. And when any group has little or no interaction with other groups, its intellectual life begins to atrophy. This was not only true in Orthodox shtetls; it is a problem in much of the Islamic world today as well as in the secular liberal university.