Imagine you were rewriting the prayer book. What would you include? The Sh’ma certainly. Kaddish, definitely. The Amidah? Sure. Aleynu,
Ad-n Olam, and basically anything that Cantor Rockman sings? Of course. But sooner or later, someone on the “New Siddur Committee” is going to ask, “What about the 10 Commandments?”
And then the room will get quiet, when everybody is struck by the fact that the 10 Commandments aren’t included in the classic siddur. And then they will turn to the rabbi, and ask, “WHY aren’t the 10 Commandments included in the siddur?” And luckily, the rabbi will have an answer, because the matter is discussed explicitly in the Talmud.
It turns out that in the Temple in Jerusalem, before our siddur text was established, the Cohanim did read the Ten Commandments, right before they recited the Sh’ma. And as the prayer service was in formation, the rabbis initially suggested that the Ten Commandents should be included, but quickly changed their mind because of “the arguments of the heretics.”
Who exactly these “heretics” were is not perfectly clear. Might have been Christians, but 2000 years ago Israel was positively boiling over with different religious sects (Gnostics, Samaritans, Hellenistic Jews, Sadducees, Essenes, a whole cast of thousands).
The argument, on the other hand, is pretty clear. The rabbis feared a religious ideology that said that only the Ten Commandments mattered—not kashrut, not sacrifice, not social and ethical legislation, not shatnez (G-d forbid), no lulav or etrog or tefillin or mezuzuah or any of the religious appurtenances that make up the fabric of Jewish life. How would the heretics justify that position? They would argue that all the people heard the Ten Commandments being given by G-d. The rest might have been invented by Moses, so were not actually divine. According to one midrash, Korach, the ultimate rebel, said to Moses, “When the Ten Commandments were given, there was no mention of challah or terumah or tithes or tzitzit. You made this all up yourself.” Maimonides was so concerned about this issue that he discouraged people from standing during the Torah reading of the Ten Commandments, because he felt that it implied that some parts of Torah are more important than other parts.
The argument was strong enough to keep the Ten Commandments out of the siddur. But attempts were made for hundreds of years to reinstitute a daily recitation. And we have archaeological evidence in the form of tefillin from the Dead Sea Scrolls that include the Ten Commandments (our tefillin today do not).
There were also some communities, for example Fostat in Egypt, which read them after services. And some authorities suggest that they should be said privately before or after services. That’s probably the reason they are included in the Art Scroll Siddur.