Why doesn't everybody know the Ten Commandments?

Answer 1

The source of the Ten Commandments is in the Jewish Torah (Exodus ch.20), from which they were adopted by Christianity and, to a significant degree, by Western society in general. However, many people are aware of these Commandments but haven't learned them well enough to know them by heart.
In Islam, the Ten Commandments are present to some degree. Several of the Ten Commandments have their parallels in passages in the Qur'an; and the Qur'an refers to the Torah (as a whole) as being God-given (for the Jews).

Other people, however, may or may not have heard of the Ten Commandments, especially in parts of the world where Christianity and Judaism constitute a small minority.

See also:

More about the Ten Commandments

Answer 2

Not everybody is a Christian or Jew. Also, not everybody who is cares a lot about religion.

Answer 3

Although most Westerners are generally aware of the Ten Commandments, not everyone is Christian or Jewish, and therefore not everyone has any real need to know the Ten Commandments.